As candidates prepare for the 2008 US presidential elections, certain issues are bound to dominate campaigning.
Here we outline the positions of those thought to be the main contenders for the Democratic and Republican parties on each of the key issues.
For the Democrats we look at New York Senator Hillary Clinton, former North Carolina Senator John Edwards and Illinois Senator Barack Obama.
For the Republicans we consider former New York Mayor Rudy Giuliani, Arizona Senator John McCain, former Massachusetts Governor Mitt Romney and former Tennessee Senator Fred Thompson.
WAR IN IRAQ
Hillary Clinton: Voted to authorise the 2003 invasion. She has not apologised for that vote but has since said that, had she known then what she knows now, she would have acted differently. She opposes the 2007 "surge" strategy of pouring more troops into Baghdad and has said she would end the war if she were president. She favours the phased redeployment of troops and caps on troops numbers in Iraq.
John Edwards: The former senator voted in favour of going to war in 2003. He has said he regrets that decision and would not have made it based on what he knows now. He favours a complete withdrawal of US troops from Iraq in 12-18 months and has said that, as president, he would end the war. He would also talk to Iraq's neighbours, including Syria and Iran.
Barack Obama: Opposed the war in Iraq from the outset and has said there is "no military solution" to the situation there. He rejected the surge strategy, saying it was time for a fundamental change in policy. In January 2007 he proposed a plan for a phased withdrawal, saying benchmarks must be set and a political solution reached.
Rudy Giuliani: The former New York mayor supports the war, saying Iraq has to be seen in the context of a broader threat to the US. He has backed George W Bush's surge strategy, but has said some way to evaluate progress in Iraq is needed. He opposes setting any kind of timetable for withdrawal, a move proposed by the Democratic-led Congress in a bill vetoed by Mr Bush.
John McCain: Voted for the 2003 invasion and has said US forces should remain until Iraq is able to defend itself. He has been a prominent supporter of Mr Bush's troop escalation, although he has criticised other aspects of the war's handling. He has warned withdrawal plans could trigger genocide in the region. The senator does not think talking to Iran and Syria will serve US interests.
Mitt Romney: Supported the initial decision to go to war but has criticised its handling. He has backed the president's surge strategy and would maintain troop levels in Iraq. He favours talking to Iraq's neighbours. Overall, he sees the conflict as part of a wider struggle, in which "violent radical jihadists" want to replace moderate Arab governments with a caliphate and destroy America.
Fred Thompson: Voted for the 2003 invasion and has said he would do "essentially what the president's doing" in Iraq, although he has also talked of rectifying past mistakes. He says he trusts Gen David Petraeus' leadership in Iraq and that the "surge" strategy should be given time to work. He sees the conflict as a front in the wider fight against "radical Islamic terrorism".
Hillary Clinton: Has said rising income inequality is undermining America's middle and working class. She wants to reduce benefits given to big companies and invest in education and creating new jobs. She has set out a 10-point plan to reform government by cutting waste and improving transparency. She has also supported women's movements for equal pay.
John Edwards: Blames Mr Bush's tax policies for increasing the tax burden on America's middle class and has pledged to try to cut costs for families, while rewarding work. He has talked about creating 1 million jobs by making the US the world leader in generating clean, reliable energy. He has also called for a national goal of eliminating poverty within 30 years.
Barack Obama: Like Mr Edwards, the Illinois senator has promised to help middle class families struggling with rising costs and stagnant pay. He has called on Washington to put the interests of big business aside and focus on reforming health care and education. He has also said the US must reshape its economy to compete in the digital age and should invest more in scientific research.
Rudy Giuliani: Has emphasised his budget-cutting record as New York mayor and promised to take the same approach as president. He has emphasised tax cuts over spending cuts to achieve a return to the party's fiscally conservative roots.
John McCain: Has put preserving America's economic freedoms at the heart of his campaign. He has talked of the need to rein in federal spending before making tax cuts, but has also promised to keep taxes low and simplify the tax code. He has drawn a link between loss of control of federal spending and the Republicans' defeat in the 2006 mid-term elections.
Mitt Romney: Intends to encourage economic growth by keeping the tax base low and simplifying the tax code. He has called for greater responsibility in federal spending. He won acclaim for taking over the organisation of the 2002 Salt Lake City Winter Olympics when planning looked on the verge of collapse, delivering a successful and profitable event.
Fred Thompson: Has said he believes in lower taxes to foster growth and promote free markets, private property and fair competition. He has talked of his efforts to balance the budget and reduce waste while in the Senate. He says welfare spending must be cut to ensure the nation's future economic security. He would keep President Bush's tax cuts.
Hillary Clinton: Made clear at the Democratic debate in South Carolina that if America were to be attacked, she would not shy from "a military response" if appropriate. She has pressed for greater anti-terror funding for New York state and other areas perceived as at higher risk of terror attack. She backed the Patriot Act in 2001, which granted the government unprecedented powers to investigate terror suspects in the wake of 9/11, and voted to reauthorise it in 2006.
John Edwards: His plans for national security focus on tightening border control and better identifying and protecting potential targets for terror attack. He wants to strengthen the military and see better emergency response plans put in place. He voted in favour of the Patriot Act in 2001. He has dismissed the idea of a "global war on terror", however, as an ideology used by the Bush administration to justify failed policies.
Barack Obama: Mr Obama, who sits on the Senate Committee on Homeland Security and Governmental Affairs, has argued for national security funding to be allocated to areas according to likely risk of attack. He has also urged bipartisan cooperation on the issue. He has criticised the Patriot Act but voted to reauthorise it in 2006.
Rudy Giuliani: In campaigning and the debates, Mr Giuliani has made repeated reference to his experience leading New York after the terror attacks of 9/11. He has said that failure in Iraq will mean a greater risk of terror attack in the US.
John McCain: A former Navy pilot who was held as a prisoner of war in Vietnam, Mr McCain has been the leading Republican voice in efforts to prevent the CIA using "cruel, inhumane or degrading treatment of prisoners", successfully sponsoring a law banning such treatment. He backed the Patriot Act in 2001 and 2006.
Mitt Romney: His record on security includes his experience as leader of the 2002 Salt Lake City Winter Olympics. He has been a member of the Homeland Security Advisory Committee. He has vowed to track down Osama Bin Laden as part of global efforts against a wider jihadist threat to the US.
Fred Thompson: Has said the US must "deploy every resource including diplomacy, intelligence, and economic power" to defend itself and its interests. He has spoken of his lead role in passing the 2002 Homeland Security bill. He would consider pre-emptive attack if Iran came close to developing nuclear weapons, but has said he favours exploring all options to deal with Tehran.
Hillary Clinton: Has said she is in favour of comprehensive immigration reform, including tightening border security, tougher penalties for employers who hire undocumented workers and a system to bring the estimated 12m illegal immigrants "out of the shadows". She has highlighted the security implications, post 9/11, of not knowing who is entering the US via its border with Mexico.
John Edwards: Has talked of the need to marry tougher border control with a policy that embraces America's history as a nation of immigrants. He has described the situation on the US-Mexico border as "a mess" and says the US needs to devote more money, technology and manpower to policing it. He proposes allowing illegal immigrants to "earn" US citizenship by paying a fine and learning to speak English.
Barack Obama: Has said the US-Mexico border must be better policed and highlighted the plight of the hundreds of illegal immigrants who die each year making desert crossings. He favours stricter penalties for employers who hire undocumented workers. He has argued that proposals to give illegal immigrants already in the US a path to citizenship are not an amnesty if they have to pay a fine and learn English.
Rudy Giuliani: Has hardened his stance on immigration since his tenure as New York City's mayor. He has opposed the bill co-sponsored by Mr McCain and has spoken of the need to have tamper-proof ID cards for immigrants and a hi-tech fence. He has said illegal immigrants should be fined, have to learn English and not be given any kind of fast track to citizenship.
John McCain: The Arizona senator has come under fire from many fellow Republicans for co-sponsoring a bipartisan bill which they say offers an amnesty to illegal immigrants. He has argued that a temporary worker programme is needed and that measures must be taken to put the undocumented workers already in the US on a path to citizenship. The borders must also be secured to ensure national security, he has said.
Mitt Romney: Has called bipartisan legislation proposed by Sen McCain and Edward Kennedy an "incredible gift" to illegal immigrants, describing it as a form of amnesty. He has stressed that people in the country illegally should not get a faster route to permanent residency than those who enter legally. He is in favour of biometric identity cards indicating people's work status.
Fred Thompson: Sees immigration as a problem "so big that it now defies a good solution". He has said US borders must be secured before new legislation is passed. He has criticised Mexico's president for opposing US plans for a fence on the Mexican border. While in the Senate, he voted to allow more temporary immigrant workers and more highly skilled workers into the US.
Hillary Clinton: Has said she wants to provide universal health care coverage. While First Lady, she oversaw a huge but unsuccessful project to give full cover to all Americans. She has said she hopes to make health care "the no 1 voting issue in the 2008 election". Under her plan, insurance companies would be required to offer insurance to everyone, while preventative efforts and reforms would lower costs.
John Edwards: Under a comprehensive reform plan, Mr Edwards aims to insure all Americans by 2012. He has said he wants businesses, government and individuals to share responsibility, make insurance more affordable and reform the market to improve choice and quality and cut costs. He has said he would roll back Mr Bush's tax cuts for the wealthiest Americans, due to expire in 2010, to help pay for the changes.
Barack Obama: Providing universal coverage is also at the centre of Mr Obama's plans. He proposes keeping the private insurance system but injecting cash to expand it to cover everyone. Insurers would not be allowed to refuse coverage because of pre-existing conditions. He predicts the typical consumer would save $2,500 (£1,260). Campaign aides put the cost at $50-65bn a year, financed largely by eliminating tax cuts for the wealthy.
Rudy Giuliani: The former mayor has said the government needs to work with private markets and give people more choice in order to improve quality and cut costs. He has said the system is flawed but should be fixed from its strengths. "We shouldn't turn it into socialised medicine," he said.
John McCain: Has highlighted his record on supporting health care for military veterans. He says he is prepared to make tough decisions to reform heath care and social security programmes and warns that without bipartisan action the costs will become unsustainable.
Mitt Romney: As governor of Massachusetts, Mr Romney backed legislation requiring everyone in the state to have health insurance or face legal penalties. He says market reforms, rather than tax increases or government programmes, are the way to extend health coverage to all Americans. He insists individuals have a responsibility for their own health care.
Fred Thompson: Has said the US has the best health care system in the world but is paying more than it should for it. He favours individuals taking responsibility for choosing their own best value health insurance, thus using market forces to drive down costs. He has criticised "socialised medicine" proposals, citing national health care systems in Britain and Canada as an example. He voted against increasing restrictions on tobacco.
Hillary Clinton: On abortion, she has supported a woman's right to choose as a fundamental constitutional right. She has also talked about the role of faith, values and education in preventing unwanted pregnancies in the first place. Her record shows she backs "sensible gun control legislation" and has been a firm supporter of stem cell research.
John Edwards: Has said he "could not disagree more strongly" with the April 2007 Supreme Court decision to uphold a ban on late-term partial birth abortion. In the Democratic debate the same month, he said he understood it was a difficult issue for America. He favours expanding federal funding for embryonic stem cell research.
Barack Obama: Speaking at the April 2007 Democratic debate, Mr Obama said he trusted women to make their own choices on abortion "in conjunction with their doctors and their families and their clergy". He has said he believes marriage is between a man and a woman but has supported civil unions for gay couples.
Rudy Giuliani: Has said he believes in a woman's right of choice but that he personally hates abortion. He has stressed that while mayor of New York, adoptions went up by 65-70% while abortions dropped 16%. He agrees with the Supreme Court decision to uphold a ban on partial birth abortion. On gay marriage, he says he supports the legal rights afforded by domestic partnerships but believes marriage is between a man and a woman.
John McCain: At the presidential debate in South Carolina, Mr McCain said he believed Roe v Wade - the landmark 1973 Supreme Court ruling that established governments lacked the power to prohibit abortions - should be overturned. He had previously said it should not be repealed because it would force women to seek backstreet abortions. He supports the Supreme Court ruling on partial birth abortion.
Mitt Romney: Supported abortion rights while governor of Massachusetts but says he has changed his mind. He has said he was always personally opposed to abortion but was motivated to change his stance on the government's role in the issue after studying medical developments like cloning. He backs the Supreme Court decision on partial abortion.
Fred Thompson: Describes himself as "pro-life" and has said the landmark Roe v Wade case was "was bad law and bad medical science". He has said he supports adult, but not embryonic, stem cell research. He is opposed to gay marriage and does not like civil unions, but thinks individual states should be left to decide on laws on the latter. He supports the death penalty.