By Dan Griffiths
BBC correspondent in Washington
When Americans go to the polls on Tuesday they will not just be choosing a president. Most of the legislative branch - including the entire House of Representatives and a third of the Senate - is also up for grabs.
Congress, where Republicans are now in the majority, is the main legislative body in the US system of government and can have a huge influence on the success of the president's agenda.
But occupants of the White House often find it difficult to win over Congress even when their own party is in power.
Congress may well have a different agenda from the White House
Representatives and senators have their own narrow constituency interests to satisfy, and that can put them at odds with the president.
Bill Clinton discovered that in 1993, when a Democrat-dominated House and Senate voted down his health care reform plans.
And earlier this year many Republicans in the House voted against President Bush's proposed constitutional amendment on gay marriage.
The Republicans have held a majority in the House since 1994 and that is unlikely to change this year.
One reason is that election rules that apply only to the House allow both political parties to draw electoral boundaries - shaping constituencies in favour of their candidates.
The practice of redistricting is highly controversial, with critics saying democracy is being undermined, but it will mean that the Republicans will hold on to their majority this year.
In the Senate things are more complicated. It currently has a slim Republican majority, but several races this year are very close - in states as far apart as North Carolina, South Dakota and Alaska.
Throw in a very tight presidential election and two possible scenarios emerge - either a Republican clean sweep or some form of gridlock involving Democratic control of the Senate or the White House.
Here is what that might mean for some of the candidates' main domestic legislative proposals.
HOMELAND SECURITY AND INTELLIGENCE REFORM
White House: George W Bush and John Kerry have both put national security and reform of the intelligence agencies at the top of their domestic agendas. Both support creating a post of national director of intelligence.
Senator Kerry has fully endorsed the recommendations of the 9/11 Commission for an intelligence director with strong budget and personnel authority. President Bush has also given his backing to the commission proposals.
Congress: The Senate and House have passed conflicting bills on intelligence reform, which have yet to be resolved. The Senate version embraces all the suggestions from the 9/11 Commission. Many in the House are concerned about transferring budgetary powers to a new director.
Politicians in Congress had set themselves the deadline of the election to get legislation to the president. But with no agreement in sight, it is likely a bill will not be agreed until next year, at the earliest.
White House: President Bush has called on Congress to make permanent the tax cuts it approved in 2001 and 2003, saying they will help create jobs.
Mr Kerry wants to repeal those tax cuts for Americans earning over $200,000 a year. He says the changes will help to pay for health care, but he would retain the cuts for the middle class. He says rolling back tax cuts for the wealthy will restore fiscal health and lead to growth.
Congress: With Republicans in control of the House, Mr Kerry would have a fight on his hands to get any repeal through.
It might not be plain sailing for President Bush either. Many in his own party approved the tax cuts, but some fiscal conservatives baulk at making them permanent.
HEALTH CARE AND MEDICARE
White House: Health care costs have soared in the US in recent years and about 45 million Americans have no health insurance.
President Bush wants to reduce that number through private insurance by helping people purchase it. Senator Kerry aims to significantly reduce the number of uninsured, partly by expanding existing government programs, and helping businesses afford insurance. Both candidates would also limit medical malpractice lawsuits.
President Bush says the Medicare overhaul enacted last year will help senior citizens pay for costly prescription drugs and give them more health coverage choices. John Kerry said the plan was a giveaway to the pharmaceutical industry and that he would do more to help push drug prices lower, including allowing the importation of low-cost prescription drugs from Canada.
Congress: Health care and Medicare are two of the most contentious issues facing Congress. The Senate Republican leader, Bill Frist, is a doctor and was a key mover behind the Medicare reforms last year. If the Republicans win a clean sweep in the elections, it is likely there will be further legislative moves to get the private sector more involved in both health care and Medicare.
Many Democrats oppose that and, if they take either the White House or the Senate, they are likely to push for greater government involvement leading to a political stalemate.
White House: President Bush has a proposal to divert to allow some Social Security taxes - paid by workers to fund payments to senior citizens - to be transferred into private investment accounts for individual. It is unclear how he would cover the transition costs if he changes the system from today's workers paying for today's retired to today's workers saving for their own retirement. The transition may cost as much as $2 trillion and the US has record budget deficits.
Mr Kerry says private accounts expose retirees to too much risk and drain the retirement system of money it needs to pay benefits.
Congress: The Democrats say President Bush wants to privatise Social Security, something the White House has always denied. But there is no doubt that the Republicans want to move towards a more market-oriented system, something many Democrats oppose.
Social security is one of the cornerstones of American public policy and any debate is likely to be heated and divided along partisan lines.