US:A plan to overhaul the immigration system in the US, legalising most of the country's 12 million undocumented immigrants, faces a crucial test in the Senate today amid signs that some Republican supporters of reform are edging away from the proposal.
As US president George W Bush urged senators to "summon the courage" to support the Bill, negotiators from both parties were working to craft amendments that could satisfy critics, some of whom believe the legislation is too liberal, while others think it is too restrictive.
After collapsing on the floor two weeks ago, the Bill made a comeback after Mr Bush said he would approve more than $4 billion (€2.97 billion) in extra funds to secure the US border with Mexico.
A vigorous campaign against the Bill, however, amplified by conservative talk-radio hosts, appears to have unnerved some Republicans who were moving towards supporting reform.
Last week, two Republican senators who helped negotiate a compromise with Democrats on the legislation - Johnny Isakson and Saxby Chambliss - said they would oppose today's motion to limit debate on the Bill. Senate minority leader Mitch McConnell, who supported immigration reform last year, now says he is undecided.
Alabama's Jeff Sessions, a leading critic of the legislation, said its opponents were determined to block a Bill he believes would encourage further illegal immigration. "We are going to use every effort to slow this process down," he said.
Failure to win the 60 votes necessary to limit debate and avoid a filibuster would doom the Bill, at least until after the summer. As next year's congressional and presidential elections draw closer, finding agreement on such sensitive legislation could become even more difficult.
Although conservative opposition to the Bill has been loudest, with many Republican activists denouncing what they see as an amnesty for illegal immigrants, some labour activists and Hispanic groups are also unhappy.
Trade unions fear that a proposed guest-worker programme would create a new underclass of poorly-paid workers with few labour rights.
A proposal to change the system under which future potential immigrants are evaluated has faced criticism because it gives more weight to skills and qualifications than to family ties.
High-tech companies, including Microsoft and Google, have lobbied for changes to the Bill so that more highly-skilled workers can come to the US.
The Bill would expand the number of visas for skilled professionals, but the companies say the limit should be raised further to ensure that the US remains competitive.
Even if today's motion to limit debate on the Bill wins the support of 60 senators, the legislation could be killed within days as about two dozen amendments are debated.
If the Senate approves the Bill, the House of Representatives will consider similar legislation.