The US Senate on Thursday formally opens debate on a landmark nuclear arms control treaty with Russia after a test vote found strong support for the accord, a top White House foreign policy priority.

US President Barack Obama has made the new Strategic Arms Reduction Treaty (START) a linchpin of his efforts to "reset" relations with Moscow, and called for the Senate to ratify the agreement before leaving for year-end holidays.

Lawmakers voted 66-32 on Wednesday to overcome a procedural hurdle and take up the pact despite fierce Republican charges -- denied by the Pentagon -- that it will cripple US missile defense plans.

That margin put Democrats within striking distance of the 67 votes needed to ratify START, and one of two lawmakers absent, Democratic Senator Evan Bayh, has pledged to back the agreement, energizing the treaty's top champions.

"We believe we should stay here as long as it takes to get this treaty ratified, and we are prepared to do so. There's no legitimate reason not to finish," said Senate Foreign Relations Committee chair John Kerry, a Democrat.

Obama's defeated 2008 Republican White House rival, John McCain, also voted in favor of moving ahead, despite past criticisms charged that the president has "overhyped" the salutary effect it will have on ties with Moscow.

The agreement -- which has the support of virtually every present and former US foreign policy or national security heavyweight -- restricts each nation to a maximum of 1,550 deployed warheads, a cut of about 30 percent from a limit set in 2002, and 800 launchers and bombers.

The accord would also return US inspectors who have been unable to monitor Russia's arsenal since the treaty's predecessor lapsed in December 2009.

Republicans have shrugged off marquee support for the pact from inside their party and called for delaying action until a new Congress convenes in January, while readying a battery of "treaty-killer" amendments behind the scenes.

But they are all-but-certain to fail in their quest to rally the votes needed to change the treaty, which Obama and Russian President Dmitry Medvedev signed in April, and send it back to the negotiating table.

In one such gambit, Republicans were to push to modify the treaty's non-binding preamble to strike language tying offensive nuclear weapons to defensive systems, an unmistakable reference to US missile defense plans.

Senator Kit Bond, the top Republican on the Senate Intelligence Committee, condemned the treaty as "giving Russia essentially a vote on our missile defense decisions" and falling short on measures to verify Russian compliance.

"There's nothing more irresponsible than for my colleagues to push to vote on a treaty of this magnitude, affecting our national defense, without airing all of the issues and getting a full debate," he said.

But the Pentagon has said the preamble and treaty overall impose no missile defense limitations, and arms control experts warn changing the preamble would force the accord back to the negotiating table, effectively killing it.

Number-two Republican Senator Jon Kyl has repeatedly said that there is not enough time left to address concerns about the treaty, and charged Wednesday it was "quite inappropriate" to squeeze it in just ahead of the Christmas holiday.

Kerry shot back that Republicans were "finding excuses" not to back the treaty, has been available for review for months, while the administration has made key documents and officials available for questioning.

Republicans dropped one delay tactic -- forcing the entire treaty to be read aloud -- after the White House and Democratic Senate Majority Leader Harry Reid blasted delays as dangerously undermining national security.

"This is a new low in putting political stunts ahead of our national security, and it is exactly the kind of Washington game-playing that the American people are sick of," said Obama spokesman Robert Gibbs.

The White House and Reid have predicted the treaty will win ratification this year in what would be a signal diplomatic victory six weeks after US voters routed Democrats in mid-term elections.

Democrats control 58 seats and need nine of the 42 Republicans to reach the 67 votes needed for ratification, but will need 14 Republicans when a new Congress convenes in January.

The Russian lower house of parliament, the State Duma, has indicated it will ratify the treaty only after the US Senate.