Obama, who postponed his holiday vacation, lobbied hard for the Senate to complete the treaty before January when Republicans increase their numbers by five and the accord's outlook would be bleak.
Weeks after Republicans routed Democrats at the polls — seizing control of the House and strengthening their numbers in the Senate — Obama has prevailed in securing overwhelming bipartisan approval of a tax deal with Republicans and getting repeal of the 17-year-old ban on openly gay military members, a crucial issue with the party's liberal base.
The White House had made steady progress in its efforts to persuade Republican lawmakers despite McConnell and Kyl's opposition.
In announcing his support, Alexander said he was reassured by a letter from Obama, in which the president reiterated his commitment to modernizing the remaining nuclear arsenal with projected spending of $85 billion over 10 years. A significant amount of that money would go to nuclear facilities at Los Alamos, N.M., and Oak Ridge, Tenn., a critical issue with Alexander and Corker.
"My administration will pursue these programs and capabilities for as long as I am president," Obama wrote in letters to Republican Sens. Alexander and Thad Cochran and Democrats Dianne Feinstein and Daniel Inouye.
Republicans and Democrats were discussing amendments to the accompanying resolution, not the treaty, that would deal with Republican problems with missile defense and build support for the agreement.
The treaty specifically would limit each country's strategic nuclear warheads to 1,550, down from the current ceiling of 2,200. It also would establish a system for monitoring and verification. U.S. weapons inspections ended a year ago with the expiration of a 1991 treaty.
All 57 members of the Democratic caucus are expected to back the treaty; Sen. Ron Wyden, D-Ore., underwent cancer surgery on Monday and is likely to miss the vote. Republicans who have previously announced they will vote for the treaty are Richard Lugar of Indiana, Olympia Snowe and Susan Collins of Maine, Scott Brown of Massachusetts and George Voinovich of Ohio.
"I think it's going to pass and more than just pass," Corker told reporters.
Announcing they would oppose the treaty were Sen. Lindsey Graham, R-S.C., and Utah's Orrin Hatch, a Republican facing re-election in 2012 and a possible primary challenge from tea party-backed candidates.
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