A landmark US-Russia nuclear arms control treaty faced a key test vote in the US Senate as early as Tuesday as President Barack Obama ramped up pressure on wary lawmakers to back the accord.

Obama, Vice President Joe Biden, and Secretary of State Hillary Clinton wooed Republicans by telephone, lawmakers said Monday, as the top US uniformed officer, Admiral Mike Mullen, wrote a letter unreservedly backing the pact.

"This treaty has the full support of your uniformed military, and we all support ratification," Mullen wrote, renewing his and the Pentagon's strong backing for the new Strategic Arms Reduction Treaty (START).

Senators were on track to vote Tuesday or Wednesday on ending debate on the treaty, a linchpin of Obama's efforts to "reset" ties with Russia, setting the stage for a ratification ballot late in the week.

The White House and its Democratic allies expressed confidence they would rally the 60 votes needed to end debate and the two-thirds majority needed to give the treaty final approval -- 67 if all 100 senators vote.

"The White House believes that before Congress leaves town the Senate will ratify the new START treaty," said Obama spokesman Robert Gibbs, who told reporters Obama was calling key senators.

And Clinton has reached out to 17 Republican senators and one Democrat, newly minted Senator Joe Manchin of West Virginia, in some cases speaking to lawmakers several times, said a US State Department official.

On Monday, Republican Senator Scott Brown of Massachusetts confirmed that he would back the treaty, and Republican Senator Bob Corker, who has played a key role in moving START forward, strongly suggested he would as well.

Speaking after a nearly-four hour debate behind closed doors to tackle sensistive intelligence issues, Corker did not rule out that some late surprise could make him vote against the treaty but said "I don't know what that could be."

Democrats easily defeated three Republican efforts to amend the treaty, as they had earlier attempts to change the document -- which would have forced a new round of negotiations with Moscow, effectively killing the accord.

Lawmakers pressed ahead with amendments to the resolution of ratification, seeking to express Republican worries notably about the treaty's possible impact on US missile defense plans without scuttling the agreement.

Corker expressed muted but unmistakable annoyance at a warning from Russian Foreign Minister Sergei Lavrov, who told the Interfax news agency that START "cannot be reopened" and any changes would kill it.

"From the standpoint of consumption here in America that probably wasn't a great comment to have made," Corker said.

The agreement -- which has the support of virtually every present and past 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.

In a direct rebuff to Republican criticisms, Mullen said in his letter that START would not cripple US missile defense plans, strengthens ties with Russia, and will hot harm the US nuclear deterrent.

The treaty "allows us to retain a strong and flexible American nuclear deterrent that will allow us to maintain stability at lower levels of deployed nuclear forces... And it demonstrates our national commitment to reducing the worldwide risk of a nuclear incident resulting from proliferation," he wrote.

Corker said that his reluctant Republican colleagues should "look at our military leaders that support this," citing "a-to-z" backing at the Pentagon for the treaty as evidence it is "in the best interest of our country."

But others said they were committed "no votes" and accused Democrats of rushing the agreement through to hand Obama a major year-end diplomatic victory.

"No senator should be forced to make decisions like this so we can tick off another item on someone's political checklist before the end of the year," said Republican Senate Minority Leader Mitch McConnell.

The Russian parliament plans to ratify the accord only after the US Senate has acted.