By Michel D. Shear & Ashley Parker The New York Times

WASHINGTON After seven months of steadily building momentum the push for a comprehensive overhaul of the
immigration system enters its most crucial phase this week in the Senate where Republicans remain divided over how much to cooperate with
President Obama as they try to repair their partys standing among Hispanic voters.
Republican leaders are betting that passage of an 867-page bipartisan overhaul will halt the embarrassing erosion of support among Latinos last year that helped return Mr. Obama to the Oval Office. But the partys conservative activists are vowing opposition dead set against anything linked to Mr. Obama and convinced that the immigration bill is nothing more than amnesty for lawbreakers.
That intraparty clash will play out for the next three weeks on the Senate floor as Republican supporters of the bill aided behind the scenes by the Obama administration seek modest changes that they hope will secure broad support among both parties. Senator Kelly Ayotte Republican of New Hampshire announced on Sunday on CBSs Face the Nation" that she would support the immigration bill calling it a thoughtful bipartisan solution to a tough problem."
At the same time conservative Republican senators led by Jeff Sessions of Alabama and Ted Cruz of Texas are preparing an onslaught of amendments that threaten to unravel the carefully crafted compromise. Their goal is to defeat the bill altogether or at the least prevent it from providing a path to citizenship for the estimated 11 million immigrants who are in this country without authorization.
It will definitely give amnesty today" Mr. Sessions said on Friday as the chamber prepared for the debate which is scheduled to begin on Tuesday.
The final vote in the Senate which is set to come by the time senators leave for their Fourth of July break could shape the future of the
Republican Party and help determine the political strength of the conservative movement heading into next years midterm elections.
If all 54 Democratic senators vote for the bill which is unlikely supporters would need six Republicans to prevent a
filibuster and pass the legislation.
For the president the passage of a bipartisan immigration bill would be a critical piece of his legacy made even more important by the Republican defeat in April of the gun-control measures he was pushing. A failure to pass the immigration legislation would reignite anger among Mr. Obamas Latino supporters and raise questions about his ability to move an agenda forward in his final three and a half years in office.
In his weekly radio address on Saturday Mr. Obama urged Congress to give him a bill to sign by the end of the summer. We know the opponents of reform are going to do everything they can to prevent that" he said. Theyll try to stoke fear and create division. Theyll try to play politics with an issue that the vast majority of Americans want addressed. And if they succeed we will lose this chance to finally fix an immigration system that is badly broken."
The legislation which would be the first major immigration overhaul since President Ronald Reagan backed amnesty for illegal immigrants in 1986 was drafted over the last several months by the so-called Gang of Eight a bipartisan group of senators. It is largely in step with the presidents goals of offering a path to citizenship for the millions of illegal immigrants already in the country while tightening border security and expanding legal entry into the country for families and workers.
But no one expects the bill to pass the Senate without some modifications. Mr. Obamas Republican allies on the issue in particular Senators Marco Rubio of Florida a member of the bipartisan group and Tom Coburn of Oklahoma warned last week that the president could lose their support without stronger efforts to secure the border as well as other changes.
Im trying to get to where I can say yes" Mr. Coburn said as senators prepared to clear the way for what could be weeks of debate in the full chamber. But the only way you can say yes and sell it to the American people is to know that the border is secure and this bill doesnt do it."
Mr. Obama will try to set the tone for the legislative fight with a speech at the White House on Tuesday. But the battle will quickly shift to the Capitol where the biggest struggle may be over how far to go in securing Americas border with Mexico against illegal crossings.
White House officials say they expect Democrats will have to make compromises on border security by accepting some amendments that tighten enforcement. But they said Democrats would resist any amendments that significantly undermine the core of the bill a certain path to citizenship for those already in the country.
We are willing to toughen up border many of our colleagues feel thats important but without forsaking our principles" said Senator Charles E. Schumer Democrat of New York and a member of the bipartisan group. He said border security demands must be reasonable so that if God forbid theres a president in 2017 who hates immigration reform it will still go forward."
Mr. Obama has stayed clear of personally directing the legislative process from the West Wing on the theory that his participation would make it more difficult for Republicans to support it. But administration officials are working behind the scenes with allies from both parties in the Senate to provide technical support as they draft friendly amendments to the legislation. They are also helping lawmakers develop talking points and prepare side-by-side comparisons to help defeat efforts intended to undermine the bill.
The process to date in the Senate and House has been marked by strong bipartisan cooperation" said Denis McDonough the presidents chief of staff. Our hope and expectation is that the momentum will carry over onto the Senate floor to produce a bill with broad support."
Yet opposition to an overhaul has become increasingly vocal. Members of the bipartisan group spent much of last week scrambling to try to shore up Republican support for the bill. Senator Lindsey Graham a South Carolina Republican and member of the group quipped that he had spent much of his time informing begging listening."
The stakes for Republicans Mr. Graham said are too high not to try to push an overhaul through. If were not able to pass immigration reform in 2013 and its the Republican Partys fault were dead in 2016" he said.
Advocates say they are pushing for a strong bipartisan vote in the Senate some have predicted as many as 70 yes votes to help propel the bill through the House where it is expected to face a tougher path.
Senior legislative aides have privately expressed annoyance that Mr. Rubio has offered potential concessions to Republicans like Senator John Cornyn of Texas whom many Democrats do not view as someone who will ever support the bill. Mr. Cornyn has said he plans to offer an amendment that would require the apprehension of at least 90 percent of all people crossing the border illegally a target that advocates say is unrealistic.
An aide to Mr. Rubio said the senator was simply trying to improve the bill. Mr. Rubio is also working on an amendment that would take the authority to produce and carry out a border security plan away from the Department of Homeland Security and place it with Congress.
On the Hugh Hewitt radio show on Tuesday Mr. Rubio said he did not think the bill in its current form had the necessary 60 votes to get through the Senate and warned that without significant border security improvements I think weve got a bill that isnt going to become law."