Democrats pressed for a final Senate vote on a nuclear arms control pact with Russia on Sunday, confident of success on one of President Barack Obama's top priorities despite Republican opposition.

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.

And they 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.

The top Democrat in the Senate, Harry Reid, was expected to file imminently for cloture to end debate, setting up a final vote on ratifying an accord seen as a major foreign policy victory for the Obama administration.

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.

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."

McConnell was chided for that decision by senior Democrats, including Reid and John Kerry, the influential Democratic chairman of the Senate foreign relations committee.

"I have great respect for the Republican leader but am deeply disappointed that he has decided to oppose the New START Treaty," Reid said in a statement.

"Respected Republican leaders including president George H.W. Bush, former Secretaries of State Colin Powell and Condoleezza Rice have joined our united military leadership, including Secretary of Defense (Robert) Gates and Chairman of the Joint Chiefs of Staff Admiral (Mike) Mullen, to call 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."

On the Senate floor, Kerry attacked McConnell's argument that Democrats were trying to rush the treaty through, pointing out that more time had been spent debating this one accord than its three predecessors put together.

"I regret that he will not support the treaty itself, but we had an understanding that was probably going to be the case. It's not a surprise," Kerry said.

"But I find it disappointing given that the entire Republican foreign policy national security experienced statesmen group that are sort of emeritus for our nation today... they all support this treaty.

"The military supports this treaty. The leader of the Strategic Command, current and the past former seven, support this treaty. The national intelligence treaty supports this treaty."

Kerry pointed out on ABC that even the general in charge of US missile defense was on record as saying in front of the foreign relations committee that the New START treaty would in no way restrain US defense capabilities.