WASHINGTON – The Senate on Thursday approved legislation that would trigger the biggest changes to U.S. immigration policy in decades, by strengthening border security, establishing a new guest-worker program and providing the means for millions of illegal immigrants to stay in the country and possibly become citizens.

The product of a tenuous bipartisan coalition that faced tough conservative opposition, the measure calls for 370 miles of triple-layer fencing along the Mexican border, a complicated three-tier system for determining who can stay and who must leave the country, and more jail cells for those awaiting deportation. It would declare English the country’s national language, a gesture that many advocates found insulting but accepted in hopes of helping millions of undocumented workers achieve legal status.

But even as the Senate approved the bill 62 to 36, the measure’s backers acknowledged that it faces formidable opposition in the House, whose political dynamics differ markedly from the Senate’s. Numerous House members insist that Congress do nothing about legalizing immigrants until illegal border crossings are dramatically reduced.

Democrats and Republicans alike said a House-Senate accord will be nearly impossible without the vigorous involvement of President Bush, who favors an approach similar to the Senate’s. The White House has already begun lobbying efforts, but it faces resistance from more than 200 House Republicans seeking re-election this fall, many in districts where the sentiment against illegal immigrants runs high.

“This is the most far-reaching immigration reform in our history,” Sen. Edward Kennedy, D-Mass., a leader of the Senate effort, said of its passage. “It is a comprehensive and realistic attempt to solve the real-world problems that have festered for too long in our broken immigration system.”

Sen. John McCain, R-Ariz., Kennedy’s partner in the effort, said more than 11 million illegal immigrants “harvest our crops, tend our gardens, work in our restaurants and clean our houses. … Some Americans believe we must find all these millions, round them up and send them back to the countries they came from. I don’t know how you do that. And I don’t know why you would want to.”

Sens. Olympia Snowe and Susan Collins, R-Maine, voted for the measure.

But opponents called the bill fundamentally flawed and predicted it will be completely rewritten by a House-Senate conference committee, which next month will begin trying to craft a compromise version acceptable to both chambers. The House in December passed a bill that dealt only with border and workplace enforcement. It would make illegal presence in the country a felony.

After mass demonstrations by immigrants in several cities and complaints by Catholic officials – plus Bush’s recent televised speech calling for a comprehensive approach that would include pathways to legal status for undocumented aliens – House GOP leaders signaled a willingness to modify their bill. But they said the Senate bill goes too far and would amount to “amnesty,” a term that many dispute, for millions of foreigners who broke the law and jumped ahead of would-be immigrants waiting for legal entry.

A possible compromise, some lawmakers said, might start with tighter border security.