Officials deny any such plan and many analysts say it does not make sense for the Kremlin to make the change right now. But the congress gives a platform for Putin, now prime minister, to re-assert his status as Russia's most popular politician.

"The authorities need to tell the nation what is happening and the United Russia congress is a good stage for that," said Boris Makarenko, deputy head of the Centre for Political Technologies, a Moscow think tank.

Sources in the lower house of parliament, where United Russia holds 315 of the 450 seats, said the congress could be used to criticize economic policy, and analysts said there could be some re-jigging of the political system.

"As far as I know, there will be changes," said Makarenko, without elaborating on what these might be.

Putin, restricted by the constitution to two consecutive terms, stepped down as president in May and was replaced by Medvedev, his long-standing protege and the man he endorsed as successor.

Putin stayed on as prime minister, creating an unusual ruling partnership in which, many analysts believe, the former president has the senior role.

A looming economic downturn could threaten this arrangement.

"Today's regime is based on the high popularity of Putin and Medvedev," independent think tank Marketing and Consulting said in written research.

"But if living standards plunge and their popularity plummets, the political system, which does not have any balancing elements or safety valves, may simply get stuck."

PUTIN COMEBACK?

Speculation about a possible Putin comeback was sparked on November 5 when Medvedev proposed extending subsequent presidential terms -- not including his own current one -- from four to six years. Parliament approved the change on the second of three readings on Wednesday.

Kremlin critics say the reform will be used as a justification for holding early presidential elections in which Putin -- wary his role as prime minister could make him a scapegoat for the downturn -- will take back his old job.

"They are afraid that if it (the crisis) goes too far, Putin will be the victim," opposition activist and former world chess champion Garry Kasparov told Reuters in an interview.

"We should expect something at this United Russia congress. I am not sure it will be a final decision but there will be good indicators of what is going to happen."

But analysts say the timing is wrong for Putin to make his return. "This is complete nonsense," said Alexei Mukhin, head of another think tank, the Centre for Political Information.

"Any attempt by Putin or Medvedev to leave their jobs in the middle of the crisis would be interpreted by the public as cowardice, the last thing either leader needs."

Surprises are still possible when both Putin and Medvedev address the three-hour congress in an upscale shopping mall 200 meters from the Kremlin.

At last year's gathering in the same location, Putin announced he would head United Russia after -- in a throwback to Soviet-era congresses -- delegates representing workers and intellectuals urged him to lead them.

The decision for Putin to lead the party gave him a power base in parliament and entrenched his status as what some supporters describe as "national leader."

Underlining his special role, Putin will this year stick to the annual ritual he established as president of holding a live, televised question-and-answer session with Russians next month, his spokesman Dmitry Peskov said.

(Editing by Mark Trevelyan)