President Barack Obama pressed the US Senate in an interview broadcast Friday to approve a landmark nuclear arms control pact with Russia before wrapping up for the year-end holidays.

And White House spokesman Robert Gibbs predicted the new Strategic Arms Reduction Treaty (START) would easily win ratification, as a pair of swing-vote Republican lawmakers announced they would back the accord.

"The START treaty is something that I absolutely think has to get done before Congress leaves for Christmas vacation," the president told National Public Radio in an exchange recorded Thursday. "It needs to get done."

"We're going to keep on working the numbers. And hopefully, we're going to be able to get it done," said Obama, who has made the agreement a centerpiece of his efforts to "reset" strained relations with Russia.

Gibbs told reporters Friday that senators would not leave before voting on START and confidently predicted "we're going to get more than 67 votes" -- the number needed to ratify the treaty.

His comments came as Republican Senators Susan Collins and Olympia Snowe said in separate statements that they would back the treaty, brightening its prospects, after the White House worked to allay their concerns about the pact.

That brought to three the number of Republicans who have publicly said they will back the agreement, though several other Republicans have hinted they will support the treaty.

Obama's Democratic allies currently control 58 Senate seats, and need nine of the 42 Republicans to back the treaty to win approval this year -- but that number rises to 14 when a new Congress arrives in January.

Republican Senator John McCain said in a speech Friday that talks between the White House and number-two Senate Republican Jon Kyl were "very close" to yielding a deal that could see the treaty voted on "next week."

But McCain, who has not formally backed the agreement, called it "a modest accomplishment" in US-Russia relations and warned that the Obama administration had "overhyped" its likely impact on ties between Washington and Moscow.

Kyl has publicly said that he has not yet made up his mind on the treaty, but his office has bombarded journalists with news articles critical of the agreement and opinion pieces urging its defeat.

Democratic Senator John Kerry, who chairs the Senate Foreign Relations Committee, said in a statement that he was working with Republicans to kick off the debate on the treaty "in the coming days."

The agreement, a key part of Obama's efforts to "reset" relations with Moscow, 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 agreement, which has broad US public support, would also return US inspectors who have been unable to monitor Russia's arsenal since the agreement's predecessor lapsed in December 2009.

"I am confident that New START will provide predictability in our relationship with Russia and thus enhance global stability, and most importantly, our national security," said Snowe.

She said concerns about verifying Russian compliance, about the future of US missile defense plans, and about the upkeep of the US nuclear arsenal "have been satisfactorily resolved."

But Snowe qualified her support by tying it to time for "sufficient debate and amendments" to the ratification resolution, a potential problem with the year-end "lame duck" session rapidly drawing to a close.

Collins praised START as helping to "achieve mutual and verifiable reductions in nuclear weapons" and said she was backing the pact after assurances from Defense Secretary Robert Gates and Secretary of State Hillary Clinton regarding efforts to reduce Russia's short-range nuclear arsenal.

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

The White House has worked for months to ease what Republicans say are their concerns: that the treaty may handcuff US missile defense plans, and that more money is needed to ensure that the US nuclear arsenal stays up to date.