The Senate on Thursday passed a sweeping immigration reform bill proposed by the bipartisan Gang of Eight senators in April.
The approval, on a 68-to-32 vote, sends the legislation to the House of Representatives, where it is expected to face strong opposition over concerns about border security and amnesty for more than 11 million illegal immigrants.
The Senate vote culminated nearly three weeks of rancorous debate, in which Republicans charged the Democratic leadership with rushing such broad and expensive legislation into law.
The bill, backed by President Barack Obama, would invest $46 billion in new funding to increase border security and revamp the U.S. visa system.
But it faces strong opposition in the House, where many Republicans oppose giving legal status and eventual citizenship to illegal immigrants. House Speaker John Boehner has said it will not even be considered in its current form.
The legislation, debated nearly nonstop since early May — first in the Senate Judiciary Committee and now on the Senate floor — was backed by President Barack Obama, who has made enactment of such a law one of his top priorities this year.
House Republicans are producing much more narrow bills that contain no steps toward legalization and eventual citizenship for the 11 million undocumented foreigners, some of whom are now raising families with American-born children.
Boehner on Thursday warned that at every step of the legislative process he would only consider bills that enjoy the support of the majority of the 234 Republicans in his chamber.
"The House is not going to take up and vote on whatever the Senate passes. We're going to do our own bill ... that reflects the will of our [Republican] majority and the will of the American people," Boehner said at his weekly press conference.
That could doom chances of Congress sending Obama a bill to sign into law bringing the 11 million "out of the shadows," where they no longer would fear deportation and could openly seek employment, attend college and serve in the U.S. military.
But many Republicans argued that the party should heal its rift over immigration legislation. Sen. Lindsey Graham of South Carolina, who helped write the Senate bill, said it was difficult to get Hispanic Americans even to listen to Republicans because many likely think they "want to deport their grandmother."
Despite earlier statements by some House Republicans that a bill could be on the floor some time in July, Boehner declined to say when the full House might debate and vote on an immigration bill, saying that he will huddle with his fellow Republicans following a July 4 recess.
One House Republican aide told Reuters that the House debate might be put off until the fall.
Nevertheless, the Senate was treating the immigration bill as a measure of historic importance.
Senate Majority Leader Harry Reid said he would take the unusual step of calling all 100 senators to the chamber for the vote, requiring them to cast votes while seated at their desks.
On most votes, senators mill around the chamber talking to each other, creating a noisy scene in the ornate chamber.
But Reid was demanding more decorum on Thursday. "This is not a vote where people should be straggling in," he said.
The Senate formally began debate on the bill on June 7 and since then most of the fight has been over whether the southwestern border with Mexico would be sufficiently secured by the legislation.
Most Republicans have argued that none of the 11 million should gain legal status or citizenship until the border was deemed fully secured.
"In the absence of a firm, results-based border security trigger, there's just no way I can look my constituents in the eye and tell them that today's assurances won't become tomorrow's disappointments," Senate Republican leader Mitch McConnell said as he outlined why he would vote against the bill.
But Democratic and Republican supporters of the bill said opponents were trying to erect an impossible standard and one that could indefinitely delay the pathway to citizenship.
On Wednesday, the Senate approved a major border security amendment aimed at broadening Republican support for the bill.
It would spend $46 billion over 10 years to place 20,000 more federal law enforcement agents at the U.S.-Mexico border, finish construction of a 700-mile fence on portions of the border and purchase high-tech surveillance equipment.
Many House Republican members have expressed a desire to vote for a bill that is tough on securing the border and cracks down on illegal immigrants in the interior of the country.
The Republican-controlled House Judiciary Committee is working on various immigration bills. This week it passed legislation requiring employers to use an electronic program known as E-verify to ensure they are only hiring legally documented workers.
The committee also is working on a bill to increase work visas for the high-tech industry, and has passed other bills to strengthen interior enforcement and to establish a new temporary work visa for farm aides.
The Senate bill contains many of these elements, in addition to the pathway to citizenship.
"What I see myself voting for, No. 1, is border security," said Republican Rep. Sean Duffy of Wisconsin. "We are a nation of immigrants, but we also are a nation of laws."
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