After eight days of amendments and fiery arguments about national identity, the Senate voted 73 to 25 to limit further debate on the bill, suggesting that it had broad bipartisan support. The Senate also defeated several last-ditch efforts to derail the legislation, and members of both parties predicted that it would pass on Thursday.
The effort to limit the tide of illegal immigration and deal with those illegal immigrants who are already in the United States will then move to negotiation between the Senate and the House, which has passed legislation that focuses on bolstering border security and offers no provision for citizenship. The gulf between the two versions is so vast, and the politics of immigration so heated in this election year, that the prospects for a deal remain murky at best.
Many House Republicans vehemently oppose the provisions in the Senate bill that would legalize most illegal immigrants and create a guest worker program that would bring 200,000 foreign workers into the country each year. They have vowed to fight to prevent the legislation from becoming law, and they have the support of many grass-roots conservatives around the country.
But supporters of the bill hailed the coalition of Republicans and Democrats that fended off conservatives' repeated efforts to kill it. Enactment of the measure would also be a victory for President Bush, who has thrown his support behind it.
In December, the House defied Mr. Bush's call for a guest worker program and passed a border security bill that would criminalize illegal immigrants' presence in the country. Representative Tom Tancredo, Republican of Colorado, said Wednesday that he and other House conservatives remained steadfast in "support for a security-first approach to immigration."
So with passage of the Senate legislation on track, senators and Bush administration officials quickly turned their attention to wooing the House. For the second straight week, the administration dispatched Karl Rove, the president's political adviser, to a meeting of House Republicans to press Mr. Bush's case for an approach broader than theirs.
In an effort to reassure conservatives, administration officials also moved swiftly to make good on their promise to reinforce beleaguered Border Patrol agents, telling the House Armed Services Committee that the first contingent of up to 6,000 National Guard troops would be deployed to the border with Mexico on June 1.
A number of Senate Republicans, including Mr. McCain, Chuck Hagel of Nebraska and Lindsey Graham of South Carolina, said that they were reaching out to their House colleagues and that some seemed interested in finding common ground.
The leader of the conservative caucus in the House, Representative Mike Pence, Republican of Indiana, proposed a bill on Tuesday that would allow illegal immigrants to become guest workers, though they would not be allowed to become permanent residents or citizens.
In addition, Mr. McCain said he had spoken to Representative Michael N. Castle, Republican of Delaware, who told him that several of his colleagues were interested in supporting a compromise.
But Mr. Castle, a moderate who supports the outlines of the Senate bill, warned that the negotiations ahead would be extremely hard and said both the measure produced by the House and the one produced by the Senate might end up being significantly rewritten.
"There are House members who think the Senate has already gone too far," Mr. Castle said. "Blending it with the House bill is going to be a very difficult process."
"I wish I could tell you that I think a majority of the House is looking for the same kind of solution" as the Senate, he said. "I couldn't say that right now."
Senators opposed to their chamber's bill said they were now placing hopes on their allies in the House.
"We've had some good debate in the Senate," said Senator Jeff Sessions, Republican of Alabama, who is a fierce critic of the measure. "But it's still not fixed, in my opinion, in a whole number of ways. What really needs to be done is for the bill to be pulled down."
Under the Senate agreement, illegal immigrants who have lived in the United States for five years or more, about seven million people, would eventually be granted citizenship if they remained employed, passed background checks, paid fines and back taxes, and enrolled in English classes.
Illegal immigrants who have lived here two to five years, about three million people, would have to leave the country briefly and receive a temporary work visa before returning, as a guest worker. Over time, they would be allowed to apply for permanent residency and ultimately citizenship.
Illegal immigrants who have been here less than two years, about one million people, would be required to leave the country altogether. They could apply for the guest worker program, but they would not be guaranteed acceptance in it.
The legislation would also require employers to use a new employment verification system that would distinguish between legal and illegal workers. In addition, it would impose stiff fines for violations by employers, create legal-immigrant documents resistant to counterfeiting, increase the number of Border Patrol agents and mandate other enforcement measures.
Critics of the bill did gain some notable victories. They won passage on amendments that call for 370 miles of fencing along the border with Mexico, designate English as the national language and reduce the number of foreign guest workers to be admitted annually to 200,000 a year from 320,000.
Still, the central elements of the bill, the legalization of illegal immigrants and the guest worker program, emerged almost entirely intact.
