WASHINGTON -- A bill that would require sweeping changes in immigration law is on track for final approval today in the Senate, setting up contentious election-year negotiations with conservative House leaders who are demanding a harsher crackdown on undocumented immigrants.

The Senate bill, which incorporates ingredients of President Bush's proposal, combines enhanced border security measures, a guest worker program, and a path to citizenship for most of the 12 million undocumented immigrants now in the United States.

The bill's supporters said they expect an overwhelming vote in favor of the bill today, with a margin similar to yesterday's 73-to-25 vote to close off debate.

The leaders said they hoped the size of the majority in the Senate will convince House opponents that any bill must include ways for undocumented immigrants to achieve legal status -- a key difference between the two chambers.

``This creates major momentum for this legislation," said Senator Edward M. Kennedy , the Massachusetts Democrat.

``Overwhelming bipartisan support for a common-sense, comprehensive immigration reform -- this may be the most important vote that we cast [this year] here in the United States Senate."

Final Senate approval will mark a victory for Bush, whose televised address a week ago last Monday called on the Senate to make such a move. The bill's supporters received 13 more votes than the 60 needed to block a filibuster by conservative Republicans -- a margin that both Republicans and Democrats said was unthinkable before Bush's speech.

Still, House leaders remained adamantly opposed to legal status for those who entered the country illegally. They label this principle ``amnesty."

The Senate bill may have to be significantly watered down before it could become law -- possibly removing the ability for illegal immigrants to eventually become citizens. If not, the entire issue could ultimately die in stalled negotiations between the two bodies.

With that possibility of an impasse in mind, a group of influential Republican senators -- including John McCain of Arizona, Chuck Hagel of Nebraska, and Lindsey O. Graham of South Carolina -- have begun meeting with rank-and-file Republican House members, to persuade them to support the Senate approach. Bush's top political aide, Karl Rove, is holding similar meetings with other House members.

``The American people are liking what they see" in the Senate bill, Graham said at a press conference. ``To those who believe that [approving] no bill is a good answer, you're dead wrong. . . . To do nothing is a political loser."

Graham and McCain said they are warning their fellow Republicans that a failure to act will reflect poorly on the GOP's leadership on an issue that is of great concern to the public.

``It would contribute to the low opinion which Americans have of Congress," McCain said at a press conference. ``We need to make progress."

House leaders have yet to bend on the issue of immigration changes as laid out in the Senate measure. Kevin Madden, a spokesman for the House majority leader, John A. Boehner , said yesterday that the chamber's leaders are keeping an open mind.

But Madden also said that everyone should be aware that the House approach is far different from the one being supported in the Senate. ``No one underestimates the challenges that lie ahead in trying to come up with an agreement," Madden said.

The bill is emerging from two weeks of Senate debate with a series of changes sought by conservatives. Senators approved a 370-mile fence along portions of the Mexican border, voted to make English the ``national language" of the United States, and voted to cap the guest worker visas at 200,000 per year, down from 400,000 in the original bill.

The original bill had no fence or English-language provisions.

Those concessions are particularly troubling because the bill is likely to become even less accommodating of immigrants in talks with the House, said Ali Noorani , executive director of the Massachusetts Immigrant and Refugee Advocacy Coalition. ``They're setting some really scary precedents," Noorani said. ``They keep toeing the line of going too far. They haven't quite crossed the line yet, but they're definitely close."

The National Network for Immigrant and Refugee Rights, a coalition of immigrants' rights groups, is opposing the Senate bill, saying it makes citizenship too difficult to achieve for undocumented immigrants. ``The rush to reach a bipartisan accord on immigration legislation has led to a compromise that would create deep divisions within the immigrant community and leave millions of undocumented immigrants in the shadows of our country," the group wrote in a joint statement. ``We say, `No deal.' "

But architects of the Senate compromise said they succeeded in turning back a series of more damaging amendments. One proposal would have prevented guest workers from ever becoming citizens. Another would have delayed any new work programs for immigrants until the Department of Homeland Security declares the nation's borders to be sealed; this would be unlikely to happen for many years.

The changes that were approved by the Senate were necessary for the sake of compromise, said Richard J. Durbin of Illinois, the Democratic whip. ``There are plenty of things wrong with this bill, but there are plenty of things right with it, too," he said.

Still, the amendments were not enough to convince a core group of conservative senators, foreshadowing the greater opposition the bill will face in the House.

Senator Jeff Sessions , an Alabama Republican, said the country can expect to add 28 million new legal residents over the next decade if the bill becomes law. This would leave taxpayers with a huge burden of services that will need to be provided, he said.

``It's still not fixed, in my opinion, in a whole number of ways," Sessions said of the Senate bill. ``It will absolutely leave us in a weaker fiscal position than we are today."

Twenty-three of the 25 votes against advancing the bill were cast by Republicans; they were joined by two Democratic senators, Robert C. Byrd of West Virginia and Byron Dorgan of North Dakota. All 12 New England senators voted in favor of moving the bill forward, as did the majority leader, Bill Frist of Tennessee.

The bill would separate illegal immigrants into three categories, based on how long they have been in the United States. Those here for longer than five years -- an estimated 7 million people -- could earn legal work status immediately and could get on a path to citizenship by paying fines and back taxes, learning English, and maintaining stable work histories and clean criminal records. Those who have been here for between two and five years -- about 3 million people -- would have to leave the country and apply for work permits at a port of entry before beginning the path to legal work status and eventual citizenship.

The 1 million to 2 million undocumented immigrants with less than two years in the United States would be on a slower path to citizenship.

They would have to return to their countries of origin and apply for standard work permits and green cards if they want to return. Unlike those who have been in the country longer, they would not receive preference for work permits.

The measure would also increase penalties for employers who hire undocumented immigrants, and would require deportation for undocumented immigrants convicted of a felony or three misdemeanors. In addition, it would more than double the Border Patrol force of 11,300 agents within the next five years.