The US Senate set its sights on passing a bill granting legal status to 12 million illegal immigrants, despite claims by opponents that the move is tantamount to a mass amnesty.
The fragile coalition of lawmakers behind the deal hopes to cling together despite an expected flurry of amendments designed to kill a measure that would form a key plank of President George W Bush's legacy.
The bill, agreed last month with the White House, would also establish a merit-based points system for future immigrants and institute a low-wage temporary worker programme.
It includes a border security crackdown, punishments for employers who hire illegal immigrants and an attempt to wipe out a backlog of visa applications from those who have gone through legal immigration channels.
Harry Reid, leader of majority Democrats in the Senate, said he hoped the deal could be put to the vote this week, then passed to the House of Representatives, which is expected to take up the issue in July.
Supporters of the deal fear the measure will die if it is not passed within months, before it gets caught up in the maelstrom of next year's congressional and presidential elections.
Colorado Democratic Senator Ken Salazar warned that the credibility of Congress was at stake over a problem cleaving American politics. "This is an issue of national security," Salazar said.
"It would be an abdication on the part of the US Senate, and Washington today, if were are not able to move forward on immigration reform," he said.