By Matt Spetalnick and Susan Cornwell
WASHINGTON (Reuters) - Strains between Washington and Moscow over Russia's disputed parliamentary elections are threatening U.S. President Barack Obama's "reset" policy, and tensions could escalate further under the glare of coming presidential votes in both countries.
Prime Minister Vladimir Putin's attack on Secretary of State Hillary Clinton for questioning the validity of Sunday's elections signaled a possible endpoint in the warming trend that Obama's aides have hailed as one of his signature foreign policy achievements.
With Obama seeking reelection in November and Putin widely expected to reclaim Russia's presidency in March, the political season is likely to be marked by a hardened tone between former the Cold War foes, analysts say.
A stronger U.S. line could help Obama counter Republican charges that his Russia policy amounts to appeasement and also give him a boost with fellow Democrats who want a more assertive approach on human rights.
Putin, a former KGB spymaster, may hope his tough talk will divert attention from his emerging domestic political problems and also appeal to voters' nationalist instincts.
"We're at an inflection point after a period of reconciliation, and it could go badly from here," said Matthew Rojansky, a Washington-based Russia expert at the Carnegie Endowment for International Peace.
The latest shift comes in the wake of a serious election setback for Putin's ruling party. Political opponents, who say United Russia would have done even worse if not for widespread fraud at the ballot box, responded with protests
Clinton's assertion that the elections were neither fair nor free drew angry accusations from Putin that she had instigated the demonstrations.
The unrest, while not likely to bring down a leader whose poll numbers remain high, has shown his grip on power to be not as solid as many had assumed.
Putin accused Clinton of encouraging "mercenary" Kremlin foes. "She set the tone for some opposition activists, gave them a signal, they heard this signal and started active work," he said.
Putin has long been known for anti-U.S. rhetoric but rarely has he singled out such a high American official for such a direct and personal diatribe.
WASHINGTON'S CALCULATED RISK
The White House stood by Clinton's criticism of the conduct of the parliamentary elections but played down any threat to overall U.S.-Russian relations.
Spokesman Jay Carney insisted that even while pursuing cooperation with Russia on issues of common interest, the United States reserved the right to speak out when it sees Russia violating human rights.
The U.S. administration had been bracing for the likelihood that Putin's return to the Kremlin next year will complicate the reset in relations Obama launched in 2009.
Washington is now taking the calculated risk that the leadership shuffle in Moscow will not roll back the biggest gains -- a new nuclear arms-reduction treaty and use of Russian territory for supplying U.S. forces fighting in Afghanistan.
Analysts believe that while Putin might try to show Russia's diplomatic clout by withholding support for any new U.N. sanctions against Iran, he is too invested in the reset to allow a rupture in relations with Washington.
With Putin considered by Washington to be the "Alpha dog" of the ruling tandem since yielding the presidency to Dmitry Medvedev in 2008 and becoming prime minister, the reset would not have been possible without Putin's backing for it.
Putin must also be mindful that Washington has been instrumental in smoothing the way for Russia's long-sought accession to the World Trade Organization.
But big differences remain over a proposed U.S. missile shield in Europe, which Washington insists is meant to protect against Iran but Moscow sees as a threat to its security.
And on top of that, the U.S. president -- known for his "no-drama Obama" persona -- is considered an unlikely match for Putin and his more mercurial personality, which may make it hard for the two to form much of a personal bond.
ECONOMY TRUMPS FOREIGN POLICY
"We're looking at a less-happy period in relations, but neither side want to let things fall apart," said James Goldgeier, dean of the School of International Service at American University in Washington.
Obama will not want to be seen closing the book entirely on the reset, which his administration has billed as a top foreign policy priority.
Foreign policy will be trumped by the economy and jobs in Obama's difficult reelection drive, but his aides want to make sure his efforts on the world stage still weigh in his favor.
Republican presidential candidates and senior lawmakers see Russia as a weak point in Obama's record, especially with Putin's expected return to the presidency.
"What has been the result of the administration's good-faith desire for a so-called reset of relations with Russia?" Republican Senator John McCain said in a speech on the Senate floor. "The answer, I'm afraid, is preciously little."
U.S. missile defense advocates on Capitol Hill are holding up Senate confirmation of Obama's nominee as ambassador to Russia, Michael McFaul, over concerns that the administration may be considering sharing technical data with the Russians about the missile shield in Europe.
McFaul was approved by the Senate Foreign Relations Committee in November. But Republican Senator Mark Kirk has demanded written assurances the administration will not provide Russia with access to missile technology, before he allows a Senate floor vote on McFaul's confirmation.
(Editing by Todd Eastham)