Updated 11:36 a.m.

Senators have reached an agreement that would almost double the number of federal agents along the U.S.-Mexico border, require construction of 700 miles of border fencing and provide money for aerial drones, according to several Senate aides who said the deal should ensure significant Republican support for an immigration measure that is expected to be approved next week.

Sen. Bob Corker (R-Tenn.). (AP)

Sens. Bob Corker (R-Tenn.) and John Hoeven (R-N.D.) later Thursday are expected to announce details of the agreement that they worked on in recent days with members of the bipartisan "Gang of Eight" who wrote the immigration bill.

"We've made significant advances," Senate Majority Leader Harry M. Reid (D-Nev.) said Thursday morning as he opened the Senate, saying he is "willing to look at any reasonable amendment" and expected to hear more from Republicans later in the day.

Corker was short on specifics Thursday morning, but said in an interview with MSNBC that the deal is so tough that it’s “almost overkill.”

In an early sign of how critical the deal could be towards securing more GOP support for the immigration bill, Sen. Mark Kirk (R-Ill.) said Thursday morning that he now plans to vote for the legislation because the new deal “will restore the people’s trust in our ability to control the border.”

The agreement calls for a "border surge" that would double the number of U.S. Border Patrol agents along the southern border to more than 40,000, according to aides. The federal government also would need to complete construction of about 700 miles of fencing along the border, essentially forcing compliance with immigration laws passed in 1996 and 2006 that authorized its construction.

Funding for the additional border agents would add roughly $30 billion to the overall cost of the immigration proposal, the primary reason why members of the bipartisan gang initially refused to engage senators on adding additional personnel. But aides said that because a  Congressional Budget Office assessment of the bill released this week showed that the legislation would reduce federal deficits by nearly $200 billion over the next decade, the gang now believed it could accept additional costs for border security.

Sen. John Cornyn (R-Tex.), who introduced a more aggressive border security amendment last week, said Thursday he is reserving judgment on the new agreement, but that doubling the size of the Border Patrol would be "a substantial movement in terms of boots on the ground."

The agreement is another victory for the bipartisan gang, but also for Corker, Hoeven and at least eight other centrist Senate Republicans who were always expected to support the immigration bill, but wanted to toughen the legislation by adding stricter border security provisions in hopes of increasing the margin of bipartisan support. Supporters believe that a significant bipartisan majority might all-but force House Republicans to take up the Senate bill even as it considers smaller, more conservative proposals.

Discussions among Republicans began late last week when Republicans realized that an aggressive border security proposal introduced by Sen. John Cornyn (R-Tex.) lacked sufficient support. Democrats and the bipartisan gang said that its strict border security requirements would make it virtually impossible for illegal immigrants to eventually begin applying for permanent legal status.

After days of talks, aides said Corker and Hoeven struck the deal midday Wednesday with gang members Sens. Charles E. Schumer (D-N.Y.), Robert Menendez (D-N.J.), Lindsey O. Graham (R-S.C.) and John McCain (R-Ariz.).

News of the deal upset several of the largest outside groups closely tracking the debate. NumbersUSA, a group opposed to the bipartisan bill and pushing for lower levels of immigration, said it would ask members to call senators in opposition to the measure.

Roy Beck, the group's president, called the new agreement "a desperate political move by pro-amnesty forces to provide cover to pass a bill that would otherwise not pass. The amendment still allows the bill to give amnesty work permits and legalization before any additional enforcement" and still allows for a "massive increase" in foreign workers.

And a coalition of liberal immigrant advocacy organizations called CAMBIO said in a statement that it is "incensed" by the agreement's increase in border patrol agents, because the new hires would cost billions of dollars "at a time when the government is cutting its budget elsewhere."

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