Lawmakers filed a blizzard of potential amendments to a bipartisan Senate immigration bill Tuesday, setting the stage for weeks of intensive debate over how to reshape the nation’s ­border-control laws.

About two-thirds of the 301 proposals came from Senate Republicans, including measures to grant Congress more authority over security along the border with Mexico, to require illegal immigrants to provide DNA samples before gaining legal status and to reduce the number of undocumented workers who would be eligible to pursue citizenship.

The amendments reflected the desires of many GOP lawmakers, who say that they will support only a comprehensive overhaul that puts a higher priority on law enforcement along the border and in the workplace. The Senate Judiciary Committee will begin considering the amendments Thursday, and the fate of the 844-page bill will be tested by a process that is likely to stretch through several days of hearings in coming weeks.

Immigration advocates fear that an extended amendment process will derail the legislation by breaking apart a fragile, bipartisan coalition of eight senators that negotiated the package over several months.

“This debate should be thoughtful and thorough. It will be arduous, and it ought to be deliberate,” said Sen. Charles E. Grassley (R-Iowa), the ranking Republican on the committee, who offered 77 amendments. “Legislation of this magnitude should be carefully considered and the consequences thought through, and I expect a robust debate.”

“This debate should be thoughtful and thorough. It will be arduous, and it ought to be deliberate,” said Sen. Charles E. Grassley (R-Iowa), the ranking Republican on the committee, who offered 77 amendments. (Cliff Owen/AP)

Although Democrats have generally been more supportive of the bill, they offered dozens of amendments, including two from Judiciary Committee Chairman Patrick J. Leahy (Vt.) that would allow same-sex foreign spouses and partners of U.S. citizens to apply for visas. Republicans have said that they will not support any comprehensive bill that includes gay rights protections.

Despite the potential pitfalls, the sponsors of the legislation expressed confidence Tuesday that it will survive, even thrive, through the amendment process. Members of both parties acknowledged that the bill is likely to grow more conservative in nature because the GOP-controlled House is unlikely to support the legislation in its current form.

“Most of the suggestions we have heard raise legitimate points and suggestions on how to improve the bill,” said Sen. Marco Rubio (R-Fla.), a member of the bipartisan group whose support is considered crucial in selling the bill to conservatives. “The good news is that . . . most conservatives are ready to support immigration reform, so long as it takes serious steps to prevent another wave of illegal immigration in the future.”

The four Democrats and four Republicans in the coalition have said that they will work together to fend off any “poison pill” amendments offered with the intent of killing the legislation. In 2007, several controversial amendments from both parties helped sink a comprehensive immigration proposal in the Senate.

What’s unclear is which provisions the group is willing to entertain. Rubio, a potential 2016 presidential candidate, has come under fire from some conservatives for his support of the legislation, which would provide a path to citizenship for an estimated 11 million illegal immigrants.

Rubio aides said he is likely to support proposals to require ­double-layered fencing along the Southwest border, increase the number of background checks for illegal immigrants and reduce the number of undocumented immigrants who are eligible for legal status.

The senator met Tuesday afternoon with more than a dozen conservative activists to discuss their concerns.

“Whatever bill makes it to the president’s desk will be different than the one we see now, and I think it will move significantly to the right,” said Alex Nowrasteh, an immigration policy analyst with the Cato Institute, a libertarian think tank.

Jeffrey Bell, policy director with the American Principles Project, said Rubio was “confident that he can get the changes, that the bill has to change somewhat and that, at the end of the day, it’s going to be okay.”

Leahy has said that he hopes the Judiciary Committee will finish the amendment process and hold a vote on the bill by the end of the month, potentially sending it to the floor for a full Senate vote in the summer.

House leaders are weighing whether to consider a comprehensive immigration bill or to break it into smaller pieces, a tactic opposed by Democrats and the White House.

President Obama has expressed confidence about the legislation’s prospects, but he has acknowledged that plenty of potential problems lie ahead.

On Tuesday, the White House released a three-minute video featuring Obama adviser David Simas, whose parents emigrated from Portugal.

“When we think about what’s at stake for immigration reform,” he says in the video, “it’s a continuation of what we’ve always been — both a nation of laws and a nation of immigrants.”

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