Updated at 8:44 a.m. on 6/20
(CNN) - A bipartisan group of Senate negotiators is expected to announce Thursday it has reached an agreement designed to bolster border security mandates in the pending immigration reform bill, according to a Senate source involved in the negotiations.
The agreement includes a “border surge” plan, which would double the size of the border patrol from its current level of 21,000 agents and ensure that there would be 700 miles of fence, sources tell CNN.
A Senate aide familiar with the negotiations cautions there could be a last-minute change, but this is what is expected to be announced.
The changes are aimed at attracting 70 or more "yes" votes for the contentious bill, which supporters hope would give it much-needed momentum in the GOP-controlled House.
Under enormous pressure from conservatives opposed to the bipartisan Senate bill, House Speaker John Boehner said Tuesday he doesn't "see any way of bringing an immigration bill to the floor that doesn't have majority support of Republicans."
Earlier Wednesday, Senate debate on the immigration bill moved slowly as the floor sat lifeless in a quorum call for much of the day. However, behind the scenes, the bipartisan Gang of Eight senators continued their push to find compromises on key outstanding issues in hopes of building broad support for the legislation.
"We're working, we're working, we're working," said Sen. Chuck Schumer, D-New York, a leader of the talks said as he hustled between meetings. "I feel like we're getting better but it's not even close to being done yet."
The group has joined with two Republican senators – Bob Corker of Tennessee and John Hoeven of North Dakota - in a last ditch effort to attract more Republicans to the bill by packaging together several amendments GOP senators want included. But whether they can stitch together a complex agreement that might win the support of a dozen or more Republicans remains to be seen.
"Last night's difficulties were resolved and now we have today's difficulties," sighed Sen. John McCain, R-Arizona, another Gang of Eight leader. McCain said they are still trying to satisfy Republicans' concerns that the bill will truly toughen security at the border and that it will prevent immigrants who are here illegally from obtaining government benefits such as subsidies to buy health insurance.
"There have always been two major issues: borders and benefits," McCain said. "We've been focusing on the border and I think we've got some agreements on the benefits."
Despite the slow pace on the floor, the Senate began voting on a handful of amendments-none of them considered major-late in day.