The Senate was expected to hold a test vote Tuesday on a nuclear arms control pact with Russia, even though top Republican lawmakers stiffened their opposition to the accord, saying it needed to be "fixed."
Leading Democrats said they believed they had enough votes to ratify the treaty, which would restrict Russia and the United States to 1,550 deployed nuclear warheads, a cut of about 30 percent from a previous limit set in 2002.
"After months of consideration and five days of open and robust debate, it is time to move forward on a treaty that will help reverse nuclear proliferation and make it harder for terrorists to get their hands on a nuclear weapon," said Senate Majority Leader Harry Reid.
Earlier Sunday, he said he hoped for "strong bipartisan support to pass this treaty" adding that it is "too critical to our national security to delay."
Democrats expressed astonishment that top Republicans continued to oppose ratification when virtually every present and past foreign policy or national security heavyweight backed the move, regardless of their political stripes.
In addition to the cuts, the new Strategic Arms Reduction Treaty (START) would also return American inspectors who have been unable to monitor Russia's arsenal since the treaty's predecessor lapsed in December 2009.
President Barack Obama won a critical victory when lawmakers voted 66-32 Wednesday to begin debate on the pact, showing Democrats within striking distance of the 67 votes needed to ratify START if all 100 Senators are present.
Senators rejected on Saturday an amendment by Republican Senator John McCain to strip out language in the preamble tying offensive nuclear weapons to defensive systems. A second Republican amendment was rejected on Sunday.
The preamble is non-binding but, because it resulted from talks between Washington and Moscow, passing either amendment would have forced the accord back to the negotiating table, effectively killing the agreement.
Just before Saturday's vote, the White House released a letter from Obama to top lawmakers reaffirming his plan to deploy US missile defense systems regardless of the treaty.
This was not enough to convince the top Republican in the Senate, Mitch McConnell, who unsurprisingly announced on Sunday that he would vote against ratification.
"I've decided that I cannot support the treaty. I think the verification provisions are inadequate and I do worry about the missile defense implications of it," he told CNN's "State of the Union."
Republican Minority Whip Jon Kyl of Arizona followed suit.
In an interview on "Fox News Sunday," he said that whether the treaty receives a vote depended on whether enough time remained before the end of session to consider Republican amendments.
"This treaty needs to be fixed," Kyl said. "And we are not going to have the time to do that in the bifurcated way or trifurcated way that we're dealing with it here, with other issues being parachuted in all the time."
Republicans were chided for that decision by senior Democrats, including Reid and John Kerry, the influential Democratic chairman of the Senate foreign relations committee.
Reid noted that respected Republicans, including former president George H.W. Bush, former secretaries of state Colin Powell and Condoleezza Rice had called for ratification of this agreement.
Democrats, including Kerry and majority whip Dick Durbin, the second-ranking Democratic senator, appeared confident they had enough Republican support to ratify the treaty even without McConnell's support.
"I think we do," Durbin told "Fox News Sunday." "We had 66 votes for those who wanted to move to this debate, and I think that we have had a debate now. I think we need to bring this to a vote."
However, Republican Senator Mike Johanns of Nebraska insisted that supporters "may well get 60 votes, but they're in trouble to get the 67 votes."