A major overhaul of US immigration law has cleared a key hurdle in the US Senate after an all-out push by President George W Bush to win over conservative sceptics.
Many Republican senators joined Democrats on Tuesday in a 64-35 vote to allow debate on the bill, which calls for a "guest worker" programme, tighter border security and - most controversially - a path to citizenship for 11-12 million illegal immigrants in the US.
Bush has lobbied hard to revive the effort, which stalled June 7 on a similar vote and is touted as the biggest revision of the US immigration system since 1986.
Despite Tuesday's vote, passage is not assured. The Senate is expected to vote on the bill itself this week. In the US House of Representatives, many Republicans have concerns about the measure.
Opponents, particularly from US states near Mexico, charge that the bill fails to give clear priority to border security, offers de-facto amnesty to illegal immigrants and needs tougher visa rules.
To help break the impasse in the emotionally charged debate, two weeks ago Bush backed a proposal to provide $4.4 billion up front for tighter border security.
"We know that there's a long debate ahead when it comes to comprehensive immigration reform," White House spokesman Tony Snow said after the Senate vote.
Hours before the vote, Bush called the bill a historic opportunity to keep the US a place where people can realize "dreams and aspirations" through hard work.
"This bill goes to the heart of our values," he said. "We have proven that our nation is capable of assimilating people. And I'm confident that we can continue to be a nation that assimilates."
Tuesday's procedural vote opened the way for senators to consider amendments, many of them designed to tighten the rules. One proposal would require all adult illegal immigrants in the US return home within two years to get permanent visas.
Bush has pushed for immigration reform for years, saying the current system is broken. He has personally telephoned key senators in recent days to make his case.
The administration says the temporary worker plan would provide US industries with labour for jobs that citizens don't want to do, but also tighten enforcement against hiring illegal workers.
Employers would be required to verify through an official database that a job applicant is in the US legally.
To get in line for US citizenship, illegal immigrants would have to pay a fine, acknowledge wrongdoing, get on probation and commit to learning English.
Opponents say the White House-backed plan is too lenient and would encourage illegal immigration and underfund border security.
"We're being promised something that the American people know that we cannot and will not deliver," John Cornyn, a Republican senator from Texas, said during Tuesday's debate.