By David Alexander

WASHINGTON (Reuters) - Secretary of State Hillary Clinton, vowing to do "whatever it takes," tried to rally Senate support on Wednesday for a new nuclear arms treaty with Russia, even as more Republicans voiced opposition to a vote before the new year.

A day after a key Senate Republican expressed doubts about President Barack Obama's drive to ratify the accord before January, White House spokesman Robert Gibbs sought to dispel the notion that the treaty had been dealt a serious blow.

Gibbs said the administration was confident the Senate would take up the strategic arms treaty before Congress ends its term this year.

"We'll have the votes to pass it," he said.

Clinton went to Capitol Hill to press senators on the urgency of bringing the deal to a vote during the final weeks of the current Congress -- the so-called "lame-duck" session.

Democrats are concerned the treaty may face more difficulty in the new year because their Senate majority will be smaller in the new Congress due to recent election losses.

"This is not an issue that can afford to be postponed," Clinton said after meeting lawmakers. "The administration ... will do whatever it takes -- literally around the clock -- to reach out, to answer questions, to have discussions."

Analysts say the START debate reflects growing polarization in Congress over public policy and U.S. national security.

Obama and Russian President Dmitry Medvedev signed the New START treaty in April to replace the previous START accord, which expired last December. The new agreement commits the former Cold War foes to cut deployed nuclear weapons by about 30 percent, to no more than 1,550, within seven years.

The accord is seen as one of Obama's top foreign policy accomplishments, a part of his effort to improve relations with Russia and his drive against nuclear proliferation. He promised Medvedev on Sunday that ratification during the final weeks of the year was a priority for his administration.

But Senator Jon Kyl, who has taken the Republican lead in negotiations with the administration over the treaty, dealt the ratification push a blow on Tuesday when he announced he did not believe there was enough time left this year to resolve the outstanding differences over START.

Kyl's comments raised questions about whether Democrats could bring the treaty to a vote since significant Republican backing is needed to muster the 67 votes it will take to ratify the treaty in the 100-member Senate.

REID 'PUZZLED'

Senate Democratic leader Harry Reid said in a statement he was "puzzled" by Kyl's comments and assured him "the Senate will be in session after Thanksgiving (on November 25) and will have time to consider and ratify it."

Obama, Vice President Joe Biden, Clinton and others are holding a meeting at the White House on Thursday to discuss the treaty with former Democratic and Republican secretaries of state, the White House said.

Clinton said Senate Foreign Relations Committee chairman John Kerry and Senator Richard Lugar, the panel's top Republican, both believed the treaty needed to be ratified before the end of the year.

"Some have suggested that we should hit the pause button, that it is too difficult to do this treaty in a lame-duck session," said Clinton, flanked by the two senators. "I strongly disagree."

Other Republicans voiced new concerns about the treaty and raised objections to considering it in the lame-duck session.

"That's always a very tight session," Senator Orrin Hatch told MSNBC television. "It's going to take more time than just a couple of days or even a week to examine the START treaty."

Senator George Voinovich said Obama's vision of a world free of nuclear weapons was "noble" but implications of the START treaty for Eastern European allies had to be considered.

"I am deeply concerned the New START treaty may once again undermine the confidence of our friends and allies in Central and Eastern Europe," he said in a speech on the Senate floor.

Former Ambassador Richard Burt, a chief negotiator on the original START treaty, said some Republicans seemed to be developing the sense "that if a Democratic administration reaches an (arms) agreement that it's automatically suspect."

The situation "is really not healthy for the conduct of a coherent U.S. foreign policy," said Burt, a Republican who now chairs Global Zero, an advocacy group that seeks the verifiable global elimination of nuclear weapons.

(Additional reporting by Thomas Ferraro, Jeff Mason and David Morgan; Editing by John O'Callaghan and Eric Walsh)