"We remain extremely confident that this is a treaty that the Senate will believe is in the best interest of our national security and in reducing nuclear tensions and in providing an important inspection regime on the Russian arsenal," White House spokesman Robert Gibbs told reporters on Tuesday. "And because of all those reasons, we think the Senate will pass and ratify the START treaty in the next day or so."

But Republican Senators Jon Kyl, Lindsey Graham and John Barrasso on Tuesday called a press conference to express their displeasure at rushing the pact through.

In the current Congress lame-duck session, the Obama administration has managed to push through a tax-cut deal and a deal repealing the ban on gays serving openly in the military.

Obama stressed as a national security imperative the passage of the new START treaty, the centerpiece of his efforts to reset relations with Russia, and has delayed his departure for Hawaii for vacation to guide the pact through this year.

If put off until early January when the new Congress opens, the pact is sure to face a dim outlook in the Senate as the Democrats' strength there is reduced from 58-42 to 53-47.

Obama signed the treaty in April with Russian President Dmitry Medvedev, which limits the deployed nuclear warheads in each state to no more than 1,550 over seven years, a cut of 30 percent over the current ceiling of 2,200 set in 2002, limits the number of deployed ballistic missiles and nuclear bombers to no more than 700 each, and sets up a mechanism for verification and inspection.

Since debate began last Wednesday, some Republicans have sought to make amendments to the treaty's preamble or text but were defeated by its proponents in Senate floor action. More than seventy Republican amendments have been filed to the chamber.

Any attempt to make changes and call for negotiations means killing of the accord, as Russian Foreign Minister Sergey Lavrov warned on Monday: "I can just emphasize that the treaty, in our view, is completely in line with the national interests of Russia and the U.S.. It cannot be reopened and become a subject of new negotiations."