Comprehensive immigration reform legislation passed a significant hurdle Tuesday when the Senate voted overwhelmingly to begin consideration of the lightning-rod bill.
Thirty Republicans, including Senate Republican Leader Mitch McConnell (Ky.), voted to take up the measure in the 84-15 vote, revealing a deep well of potential support.
The vote tally was a promising sign of bipartisanship and the legislation appeared to have strong momentum after Speaker John Boehner (R-Ohio) predicted Tuesday that immigration reform would become law by year's end.
The bill will first have to emerge from the Senate, however, where many of the Republican "yes" votes on Tuesday warned they would not support the measure in a final vote unless its border security language was strengthened.
It also remains unclear whether an immigration reform bill will emerge from the House, where there is strong opposition among conservatives to the Senate bill.
Still, Boehner's comments highlight the pressure on the House — and on Republicans — to not be seen as obstacles to a bill despite opposition from grassroots conservatives.
The GOP senators who voted "yes" on Tuesday included three of the four Republicans who helped draft the measure: Sens. Jeff Flake (Ariz.), Marco Rubio (Fla.) and Lindsey Graham (S.C.). Sen. John McCain (Ariz.) missed the vote.
The other GOP "yes" votes were McConnell and Sens. John Cornyn (Texas), Kelly Ayotte (N.H.), Deb Fischer (Neb.), Rob Portman (Ohio), John Thune (S.D.), Orrin Hatch (Utah), Jeff Chiesa (N.J.), Roger Wicker (Miss.), Pat Toomey (Pa.), Lamar Alexander (Tenn.), John Hoeven (N.D.), Richard Burr (N.C.), Rand Paul (Ky.), Susan Collins (Maine), Jerry Moran (Kan.), Roy Blunt (Mo.), Dan Coats (Ind.), Dean Heller (Nev.), Ron Johnson (Wis.), Bob Corker (Tenn.), Mike Johanns (Neb.), Johnny Isakson (Ga.), Tom Coburn (Okla.), Saxby Chambliss (Ga.), Lisa Murkowski (Alaska) and Chuck Grassley (Iowa).
Even before the vote, leaders were jockeying for leverage on amendments to rework the fragile, 1,000-page compromise hammered out by the Senate's Gang of Eight.
"The bill has serious flaws," McConnell said before the vote. "I'll vote to debate it and for the opportunity to amend it, but in the days ahead there will need to be major changes to this bill if it's going to become law."
McConnell called for changes to strengthen the border security language and restrict federal benefits and tax breaks for immigrants receiving provisional legal status.
Boehner called for similar changes in an interview aired Tuesday.
"I've got real concerns about the Senate bill, especially in the area of border security and internal enforcement of the system. I'm concerned that it doesn't go far enough," Boehner told ABC.
Many Republicans are pushing for an amendment sponsored by Cornyn that would require 100 percent monitoring capability and a 90 percent apprehension rate of illegal entrants along the Southern border before granting permanent legal status to millions of immigrants in the country illegally. Cornyn's plan would also require tracking visa exits with biometric data at certain air and sea ports.
McConnell praised it as "the key amendment" and said it would "put us in a position where we can look the American people in the face and say we are going to secure the border."
Senate Majority Leader Harry Reid (D-Nev.) warned its adoption could sink the entire bill.
"He has set out the principles of what's in that amendment, and his principles would be a poison pill to this bill," Reid said.
Other members of the Gang of Eight, which drafted the legislation, disagree with Reid.
"There's been some talk about some kind of poison pill. That's not the case. Cornyn's trying to get language that he can support so we're working with him," said Flake.
Reid has claimed it will be fairly easy to round up 60 votes for Senate passage because Democrats expect only one or two defections and four Republicans on the Gang of Eight have already pledged their support.
Ayotte, for example, declared her endorsement before the procedural vote.
"Our immigration system is completely broken," Ayotte said on CBS's "Face the Nation" Sunday. "This is a thoughtful, bipartisan solution to a tough problem. So that's why I'm going to support it."
But Cornyn says his amendment will need to be adopted to push the bill over the hump.
"I've talked fairly regularly with Sen. [Charles] Schumer [(D-N.Y.)] and conversations continue," he said, referring to another Gang of Eight member.
"I think if they had 60 votes to pass the bill out of the Senate they probably wouldn't be talking to me," he added. "Which tells me they view this as a way to get it out of the Senate on a bipartisan basis that would give it some momentum and increase the likelihood of the bill passing in the House as well."
President Obama, for his part, warned opponents not to use "procedural games" to stop it in remarks on the bill Tuesday.
He said there was "no reason Congress can't get this done by the end of summer."
Rubio, a pivotal member of the Gang of Eight who is in charge of selling the bill to conservatives, has also called for changes to bolster the enforcement provisions.
Rubio said he would introduce his proposal "soon." It would include a specific border security plan in the legislation and leave less to the discretion of Homeland Security Secretary Janet Napolitano.
"The one that I've been focused on the most is that people just want to make sure whatever the border security plan is an effective one, and they're worried the Department of Homeland Security will not craft a border security plan that does that," he said.
On Tuesday, Rubio introduced an amendment to strengthen the requirement that immigrants demonstrate English proficiency before receiving permanent legal status.
Rubio's amendment would strike language in the pending bill allowing the English proficiency requirement to be met simply by signing up for a language course.
Critics of the legislation have panned it for not doing enough to require millions of illegal immigrants to learn English as a condition for obtaining permanent legal residency.
ProEnglish, a group advocating for strong language requirements for immigrants, argued the authors of the bill have overstated its English standards.