WASHINGTON (AFP) - President Barack Obama's "reset" of US-Russia ties marks one of his most tangible claims to foreign policy success, but its longevity will be tested when Dmitry Medvedev exits the presidency.

Medvedev, a valued Obama partner, said Saturday he would step aside in favor of his political mentor Vladimir Putin, almost certainly guaranteeing the current prime minister a return to the presidency in March.

"Obama and Medvedev have established a very good working rapport that has facilitated a pretty productive relationship," said Matthew Rojansky of the Carnegie Endowment for International Peace.

"That will be gone if Medvedev is no longer the formal head of Russian foreign policy," Rojansky said.

Two youthful leaders, Obama and Medvedev always seemed to see something of themselves in the other.

Both lawyers, with technocratic tendencies, they were part of a political generation that practiced statecraft on Twitter and Facebook.

But some in Washington, especially conservative Republicans, see the wily Putin as more from the Soviet era than the social media age.

The White House, conscious of chummy ties which soured between ex-president George W. Bush and once and future Russian president Putin, always insisted that the "reset" is based on national interest, not personalities.

Officials argue the outreach is sufficiently robust to outlast current political players and dismiss criticism they backed the wrong horse by courting Medvedev.

They also say the initiative made the world safer by cutting nuclear stocks and secured Russia cooperation -- or at least an absence of obstruction -- on key US diplomatic priorities such as Iran and Libya.

"For nearly three years, we have deepened our ties with Russia's institutions and people as well as its president," said National Security Council spokesman Tommy Vietor.

"Therefore, we are quite confident that we can continue to build on the progress made during the Obama administration," Vietor said, in an unusually detailed statement reflecting the importance of the reset to the White House.

Vietor stressed that Obama, who saw the need to purge growing mistrust between two former Cold War foes when he came to power in 2009, had tried to build governmental and societal ties with wider Russia, not just the Kremlin.

Obama and Medvedev, though, rarely passed up a chance to tout their friendship, swapping birthday greetings by phone and once stole out of the White House to chow down at a hamburger bar.

The guessing game will now begin over how Putin, who could rule until 2024, will approach his second bite at US-Russia relations.

If, as many believe, he was the power all along behind Medvedev, Putin must have signed off on warming ties with Washington.

Vietor stressed in his statement that "while we have had a very strong working relationship with President Medvedev, it's worth noting that Vladimir Putin was Prime Minister throughout the reset."

But a hamburger summit between Obama and Putin seems unlikely, and if Putin does return to power, a more frosty formality might also make a comeback.

Politics in the United States, as well as Russia, could challenge the "reset."

Obama, his popularity tumbling as the economic recovery withers, faces a tough reelection bid next year.

Republican presidential candidates have yet to flesh out their foreign policy visions, but a new president may be cooler towards Moscow.

Some Republicans view Putin's KGB pedigree with suspicion, suspect authoritarian motives and bemoan an erosion of freedoms under his watch, and do not absolve Medvedev of blame.

"It is time for President Obama to wake up and recognize the brutal nature of the regime he is dealing with in Moscow and to rethink his 'reset' policy,"

leading Republican lawmaker Ileana Ros-Lehtinen said last year.

The White House has spoken out against corruption in Russia, notably through Vice President Joe Biden, but says the reset has yielded demonstrable benefits.

Russia has permitted up to 50 percent of supplies for the US war in Afghanistan to be tracked on a new supply route across its territory.

Moscow also supported, or just as importantly did not block, several US diplomatic priorities in the United Nations Security Council, including Iran sanctions and a UN mandate for Libya action.

But along with China, it balked at imposing Security Council sanctions on Syria over its protest crackdown.

Tensions have also simmered over US missile defense plans, but the two sides did conclude a new Strategic Arms Reduction Treaty which cut nuclear arsenals.