WASHINGTON -- The chances of the Senate passing a comprehensive immigration bill improved dramatically Thursday after a bipartisan group of senators reached an agreement to double the number of Border Patrol agents and add 700 miles of fencing along the U.S.-Mexico border.

The deal is expected to lure previously reluctant Republican senators, who said they couldn’t support the bipartisan “Gang of Eight” bill unless it included tougher action to secure the border with Mexico.

“We’re extremely enthusiastic that a bipartisan agreement is at hand,” said Sen. Charles Schumer, D-N.Y., a Gang of Eight member. “We are on the verge of a huge breakthrough on border security. With this agreement we believe we have the makings of a strong, bipartisan, final vote in favor of this immigration reform bill.”

Even as the agreement was being drafted into amendment form, its authors predicted it will be the salve that delivers Republican votes and perhaps serves as a selling point to Republicans in the House of Representatives, where immigration restructuring faces a tougher challenge.

“I think this amendment has the ability, if passed, to bring a bipartisan effort behind reform that would then send the bill to the House,” said Sen. Bob Corker, R-Tenn., who co-authored the compromise with Sen. John Hoeven, R-N.D. “I don’t know anybody that has proposed a tougher measure, when you look at it all combined.”

Earlier this week, House Speaker John Boehner, R-Ohio, indicated that he’d adhere to the so-called “Hastert Rule” – named after former House Speaker Dennis Hastert, R-Ill. – and not bring an immigration bill to the House floor unless it had the support of a majority of Republicans.

At his weekly news conference, Boehner said that for an immigration overhaul bill to pass his chamber it had to ensure “that our borders are secure” and give "confidence that those who came here illegally are not given special treatment, that hardworking taxpayers are being respected and confidence that a majority of both parties have had their say and support the final product.”

The Corker-Hoeven plan proposes a “border surge” that would increase the number of Border Patrol agents along the U.S.-Mexico line from 20,000 to 40,000, along with requiring the additional miles of fencing.

The price tag for the additional agents, fencing and the purchase of unmanned aerial drones to patrol border activity comes to about $30 billion, lawmakers said.

A Congressional Budget Office report released this week said the Gang of Eight immigration bill would reduce federal budget deficits by almost $200 billion over the next 10 years. The report helped fuel the “border surge” idea, Schumer said.

“We didn’t have the CBO report. We didn’t know we had the dollars,” Schumer said. “We have them now. And we still keep to our goal of not costing the treasury a nickel. The CBO report was a true game-changer.”

Corker predicted that his proposal will attract at least 15 Senate Republicans. It got two Thursday: Mark Kirk of Illinois and Dean Heller of Nevada. Kirk voted against bringing the immigration bill to go to the Senate floor last week “because it didn’t have a strong border-security provision.”