Americans are deeply divided about how to deal with the estimated 12 million illegal immigrants in the United States, making immigration and border control key issues.
The debate has drawn in everyone from citizen activists to the president of the United States. Here are some of the key players:
GEORGE W BUSH, US president
President Bush has been urging legislators to reform the country's immigration laws, calling for a guest-worker programme to allow people with needed skills to live and work in the country temporarily.
Mr Bush has also backed the suggestion that illegal immigrants who are already in the country should have a way to become citizens, despite stiff opposition from some in his own Republican party who see it as being too soft on lawbreakers.
He welcomed as a "good step" an agreement reached in May 2007 by senators from both parties and White House officials which could offer legal status to many illegal immigrants as well as tightening border controls.
"This is a bill where people who live here in our country will be treated without amnesty but without animosity," Mr Bush said at the time.
The measure stalled in the Senate on 7 June when senators failed to agree to move to a final vote on the issue amid a raft of amendments. However, senate leaders have agreed to revive talks on the bill.
The proposals must be passed by the Senate and House of Representatives before being sent to Mr Bush to be signed into law.
The issue is politically awkward for Mr Bush's party, because it brings into conflict two of its core supporters - the business lobby and social conservatives. However, he has appealed to Republican lawmakers to back the reforms.
JOHN McCAIN, Republican senator of Arizona
A contender for the Republican nomination in the 2008 presidential campaign, Mr McCain launched a national campaign to allow illegal immigrants to stay in the country legally as guest workers who could earn US citizenship.
Mr McCain argues that an overhaul of US immigration laws and forging a path toward legal status for illegal immigrants currently living in the US is the best option for the country on national security, economic, and humanitarian grounds.
He believes that granting legal status to undocumented workers will help drive the nation's economy.
He backed the latest proposals and was a co-sponsor of the Senate bill put forward last year that would have allowed a guest-worker programme and a path to citizenship for illegal immigrants, but stalled in the House.
Some of his supporters worry that his policies will alienate him from the party's conservative voters.
EDWARD KENNEDY, Democratic senator of Massachusetts
Mr Kennedy, a veteran Senate liberal, was a strong supporter of the 1965 Hart-Celler Act which dramatically changed US policy by abolishing national quotas on immigration.
He was the lead negotiator for the Democrats in the bipartisan talks which produced the latest bill.
The proposed legislation was "the best possible chance we will have in years to secure our borders and bring millions of people out of the shadows and into the sunshine of America", he said.
After the latest developments, Sen Kennedy insisted that the fight would go on to seek the bill's approval.
Last year, he publicly condemned Republican legislation that would forcibly expel millions of illegal immigrants.
He was a co-sponsor, with Senator McCain, of the bill put forward last year which stalled in the House.
HARRY REID, Democratic senator of Nevada
Democratic Senate Majority Leader Harry Reid has played a major role in orchestrating the debate on the latest proposed immigration legislation.
He pulled the bill from the Senate floor after it failed to win enough Republican support to move to a final vote in early June and criticised Mr Bush for failing to press his party to back what he called "the president's bill".
Senator Reid has agreed with Republican Senate leaders to bring the bill back for further discussion. Some critics, however, say he pulled it from the Senate too soon last time and question his commitment to reform.
Senator Reid has said he wants a law that "makes our country safer, treats people with dignity, and keeps our economy moving strong".
JOHN CORNYN, Republican senator of Texas
John Cornyn has been among the staunchest opponents of the latest immigration reform legislation in the Senate.
Among the amendments he has put forward was one which would have prevented hundreds of thousands of illegal immigrants gaining legal status on the grounds of certain criminal convictions.
Under his proposal gang members, terrorists and other convicted criminals would have been denied a visa - but so too would anyone convicted of disobeying a deportation order or using false documents in the US. The amendment was defeated in the Senate.
The senator has raised serious doubts about the federal government's ability to secure US borders and enforce immigration laws and warns that the American people are also "profoundly sceptical".
He has said the bill would benefit from "more sunshine, public scrutiny, and a full and fair debate".
TOM TANCREDO, Republican congressman of Colorado
Republican presidential contender and chairman of the House Immigration Reform Caucus, Rep Tancredo has campaigned throughout the US in support of tighter border controls and immigration reduction.
He has criticised Mr Bush, saying that "with his support for open borders and an amnesty programme, it is the president who is out of step with his own party".
In televised debates with other 2008 Republican contenders, he has expressed some of the toughest views on immigration.
The Colorado congressman has called for stricter enforcement of immigration laws and suggested the US could deport undocumented workers.
JEFF FLAKE AND LUIS GUTIERREZ, Congressmen
Jeff Flake, a Republican from Arizona, and Luis Gutierrez, a Democrat from Illinois, introduced a joint bill to the House of Representatives in March 2007.
The legislation would boost border security and improve enforcement of employment law at the same time as creating a workable guest worker system, they said.
Mr Flake, who sits on the House Judiciary Committee's Immigration and Border Security Subcommittee, has said the current US immigration laws are "at odds with reality".
Mr Gutierrez said the proposed legislation would allow the US "to better protect our borders and reform an immigration system that is hampering businesses, hurting families and harming communities".
JUAN JOSE GUTIERREZ, Latino Movement USA
Juan Jose Gutierrez, national director of Latino Movement USA, has been one of the most outspoken campaigners for illegal immigrants' rights.
In 2006, he was a key organiser of the nationwide 1 May boycott, designed to show the value of immigrants to the US economy, and a mass rally in Los Angeles.
Mr Gutierrez has warned that current US immigration policy condemns illegal immigrants "to live and work in the conditions of modern-day slaves".
Arguing that undocumented workers create a net gain for the US economy, he says they should be given a chance to gain full citizenship.
PUEBLO SIN FRONTERAS, activist group
Chicago-based rights group Pueblo Sin Fronteras backed last year's immigration reform bill put forward by Senators McCain and Kennedy.
Campaigners urged all legal migrants to apply for citizenship, register to vote and then use their voice to demand legalisation for illegal immigrants.
Its president, Emma Lozano, has said US citizens must realise the immigration debate "is about real, human families, and their struggle to stay together".
THE MINUTEMAN PROJECT, activist group
US citizen activists calling themselves the Minutemen are taking direct action to stop what they call the "human flood" of illegal immigrants.
Named after militiamen of the American Revolution, the group recruits volunteers to "patrol" the border and report illegal entrants to the US Border Patrol.
Many liberals and Hispanic open-border supporters have condemned the movement as fuelling xenophobia and encouraging vigilantism. Minutemen leaders say their members abide by the law.