MOSCOW (Reuters) - Dmitry Medvedev's future role as Russia's prime minister is looking less secure after his long-time ally, Vladimir Putin, hinted that his appointment may not be a done deal.
The two leaders have agreed to swap jobs after a presidential election next year under a deal which involves Medvedev stepping aside as president at the end of his four-year term and taking Putin's place as premier.
But Putin signalled in an interview on Monday, almost in passing, that Medvedev's legitimacy to lead the government could be affected by how well the United Russia party fares in a parliamentary election on Dec. 4.
Medvedev is top of the party's list of election candidates so its performance will reflect on him personally. It is sure to win the election, but opinion polls suggest it may struggle to retain its two-thirds majority in parliament's lower house.
"United Russia is losing some ground due to it being paralysed. Medvedev's position is tied closely to United Russia and he needs to inject some energy into it," said Nikolai Petrov of the Carnegie Moscow Centre think tank.
"Putin feels obliged to Medvedev who, if he has not exactly performed well as president, has performed loyally. That's why he does him no direct harm but it is now up to Medvedev whether he survives."
Both men have both used television interviews in the past two weeks to justify their decision to carve up power between them and reassure investors and the public that Putin's return -- for 12 years if he wins the maximum two successive terms -- will not mean political and economic stagnation.
They have also sought to convince Russian voters that democracy is not at stake and that their votes count. The parliamentary and presidential elections, they say, will be fully free and the outcome is not pre-ordained.
It may be in this light that Putin, 59, suggested on Monday that Medvedev's political future could depend on the election to the lower house, the State Duma.
"If the voters vote for this (United Russia election) list and we manage to form an effective parliament in which United Russia retains its leading position, then -- building on this parliament, relying on this victory -- Dmitry Anatolyevich (Medvedev) will be able to form an effective government," Putin said.
Most political analysts and commentators say it seems unlikely at this stage that Medvedev will not become prime minister, but it cannot be ruled out.
They say there are greater doubts over how long he will be able to hold on to the post, even though his popularity ratings are strong and not far behind Putin's.
Opinion polls show support for United Russia is much higher than for any other party and its potentially strongest liberal opponents are barred from running in the election. The other parties in parliament do little to challenge Putin's authority and most media are in thrall to the Kremlin.
But opinion polls and recent regional election results have indicated United Russia could have trouble keeping the two-thirds majority needed if it wants to change the constitution.
Asked what would happen if United Russia did less well than expected in December, political analyst Dmitry Oreshkin said: "Medvedev will be guilty as he is responsible for United Russia. And as he'll be guilty, how can be trusted as premier?"
United Russia seems to have been caught off guard by the job swap announcement at a party congress that contained a broad discussion of policy and speeches by Medvedev and Putin but did not agree on a detailed election campaign programme.
The programme the party has since worked out is short on detail and based entirely on Putin's speech at the congress. A poster appeared by the roadside last month showing Putin's face, not Medvedev's.
United Russia officials dismissed any suggestion they had been taken by surprise.
But Yevgeny Minchenko, director of the International Institute of Political Analysis think-tank, said: "The main problem is not that Medvedev's image corresponds badly with United Russia's. It's just that everything was prepared for Putin as the head of the list of election candidates."
"Now they have to redo the campaign material. The mockups were ready, and the scripts for the campaign ads, and suddenly it turned out Medvedev and not Putin was leading the list."
Another potential problem for Medvedev, 46, is the threat posed by former Finance Minister Alexei Kudrin, who covets the premiership.
Kudrin was forced out of office last month after receiving a public dressing-down by Medvedev for openly criticising the president's plan to spend 20 trillion roubles ($647 billion) in the next 10 years on modernising the army.
Kudrin, 51, underlined his differences with Medvedev over spending on Tuesday in a speech to investors, and in an article for the Kommersant business newspaper which set out an action plan for fending off any future global financial crisis.
"Analysing our fiscal plans, investors see perfectly the crossroads we are at -- we will either have to cut spending or raise taxes," Kudrin wrote.
Although Kudrin has been stripped of his role, he is -- like Medvedev -- a long-time Putin ally and the prime minister has said he will remain a member of his broader team. Such comments by Putin, and Kudrin's sniping, can hardly boost Medvedev's confidence.
Additional reporting By Maria Tsvetkova; Editing by Jon Boyle