Senators formally announced a compromise amendment on border security provisions Thursday designed to woo GOP support for a comprehensive immigration reform bill, hours after lawmakers voted to set aside a more stringent border measure that Democrats had dubbed a “poison pill.”

Republican Sens. John Hoeven, N.D., and Bob Corker, Tenn., worked with Gang of Eight Sens. Chuck Schumer, Bob Menendez, John McCain and Lindsey Graham to draft the "border surge" plan, which would double the size of the border patrol, require 700 miles of border fencing, and require a detailed comprehensive south border security plan including high-tech protections.

“We have, if this legislation passes in the form that it is, with the amendment as we’ve agreed, we have secured the border,” Corker said on the Senate floor.

Democrat Chuck Schumer, who helped draft the original bill and worked on the border compromise, said the amendment "answered every criticism" of the legislation.

"It is safe to say: This agreement has the power to change minds in the Senate," he said.

The language of the amendment is currently being vetted, the senators said, and is likely to be introduced later today. Others signed on the legislation include the Republicans in the Gang of Eight as well as Republican Sens. Kelly Ayotte of New Hampshire and Dean Heller of Nevada.

Earlier Thursday, the Senate voted to table – or effectively kill – the legislation by Republican Sen. John Cornyn of Texas that would have instituted new border metrics and required those goals to be met before undocumented workers can apply for green cards. Democratic leaders called that amendment a "poison pill," and Vice President Joe Biden told attendees at a Hispanic prayer breakfast Thursday that the Cornyn measure was "one of the meanest amendments that will come up in the Senate."

Cornyn said on the Senate floor that his amendment would provide promises of results rather than “false promises” about border security goals.

Sen. John Cornyn, pushing for his border security amendment Wednesday, compares the Gang of Eight's immigration plan to an illegal immigrant amnesty ruling from 1986.

But supporters of the new compromise say that its provisions also offer assurances of “dramatically” beefed-up border security and would prevent future waves of illegal immigration.

"For people who are concerned about security, once they see what is in this bill, it’s almost overkill,” Corker said on MSNBC's "The Daily Rundown" Thursday morning.

Sen. Marco Rubio of Florida, a key Republican on the Gang of Eight who has been pushing for more border security additions to the legislation he helped draft, backed the amendment in an interview on FOX News, calling it "a dramatic expansion and improvement in border security that I hope will allow finally for this legislation to have the support it needs."

The additional provisions will come with a price tag. Aides estimate that the doubling of the border patrol agents -- just one piece of the "border surge" -- will add about $30 billion to the overall cost of the bill, though the Congressional Budget Office has offered no formal accounting of the measure yet.

Supporters of the legislation argued last week that increasing border patrol agents would be unnecessarily costly. But after a Congressional Budget Office report estimated that the flow of undocumented immigrants into the country would only be marginally decreased under the original legislation – and without another workable alternative to build conservative support for the bill – backers accepted the increase as necessary and affordable because of the bill’s other estimated positive effects on the economy.

“I can tell you this: It’s money well spent because it makes the border more security because it helps us with our sovereignty,” said Sen. Lindsey Graham, R-S.C.

The compromise talks were aimed at winning over enough Republican votes to pass a comprehensive immigration reform bill with an overwhelming majority of senators -- without angering Democrats who want to make sure that the path to citizenship included for undocumented immigrants isn't affected. A strong margin of victory for the legislation in the Senate is seen as heightening pressure on the more conservative House of Representatives to adopt the landmark immigration law.

"We have some people in our caucus that are never going to vote for an immigration bill, OK, I don't care if you -- it's just never going to happen," Corker told reporters Wednesday. "And so we realize that. And yet there are people who, with the right provisions, would."

There are signs that the strategy is working. Sen. Mark Kirk, an Illinois Republican who voted against an initial measure to bring the bill to the Senate floor, said Thursday that he is now prepared to support the immigration bill if the border security amendment is included.

Gregory Bull / AP A U.S. Border Patrol agent monitors the border structures separating Tijuana, Mexico, from San Diego. Illegal immigration into the United States would decrease by only 25 percent under a far-reaching Senate immigration bill, according to an analysis by the Congressional Budget Office that also finds the measure reduces federal deficits by billions.

"This bipartisan compromise will restore the people's trust in our ability to control the border and bring 525,000 people in Illinois out of the shadows.," Kirk said in a statement. "Once the Senate adopts our amendment, I will be proud to vote for a bill that secures our border and respects our heritage as an immigrant nation."

There's a small universe of Republicans that have also given early indications that they'll support the bill -- the four Republicans in the Gang of 8; New Hamsphire Sen. Kelly Ayotte; and Sen. Lisa Murkowski of Alaska. Republicans including Corker, Sen. Lamar Alexander and Georgia Sens. Johnny Isakson and Saxby Chambliss have also been considered possible supporters.

But Senate leaders and the Gang of Eight negotiators have been hoping to draw half the Republican conference to support the comprehensive legislation.

The most vigorous opponents of the compromise bill maintain that the Hoeven-Corker measure still does not do enough to prevent illegal immigration. The political arm of the Heritage Foundation, led by former Sen. Jim DeMint, says it opposes the amendment because it "fails to take a security-first approach," and immigration limitation group NumbersUSA is urging its members to lobby against the measure.

Speaking in opposition to the amendment, Sen. Ted Cruz of Texas called it a “fig leaf,” and Sen. Jeff Sessions said it would merely “throw money at the border” without fixing the problem of illegal immigration in the future.

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