By Christian Lowe and Olesya Astakhova
MOSCOW (Reuters) - Soon after Vladimir Putin is re-elected on Sunday, his thoughts will turn to the question that is likely to dominate his next term as Russia's president: what will he do when it ends?
Putin's victory in the presidential election is not in doubt as his ratings are high and he has the state machinery behind him, but how long the man who has dominated Russia for nearly 18 years wants to stay power is uncertain.
The constitution limits the president to two successive terms, obliging him to step down at the end of his mandate -- as he did in 2008 after serving two four-year terms.
His mandate will not expire until 2024 but the problem needs immediate attention because the uncertainty about his long-term future is a source of instability in a fractious ruling elite that only he can keep in check.
"The Russian political scene is entering a new phase," said Gleb Pavlovsky, a former Kremlin adviser who is now critical of the country's leadership. "Most discussion within the ruling elite focuses not on the next stage of the Putin era but on what will constitute the post-Putin era."
Vygaudas Usackas, the European Union's ambassador to Russia until last October, says the stakes are high.
"This is a risky moment for the system," said Usackas, who is now director of the Institute of Europe at Lithuania's Kaunas University of Technology.
Putin has at least three main options. He could take a leaf out of Chinese President Xi Jinping's book and seek an end to term limits, hand over to a place-holder for a term and then return, or anoint a successor and bow out of public life.
Each choice carries risks and Putin may have other options up his sleeve. A former spy, he is secretive and likes to pull surprises. But the uncertainty about his plans is potentially more destabilizing than anything for the ruling elite, the political, security and business leaders around Putin.
Two sources close to the Kremlin said there was, as yet, no plan for when Putin's term ends. The matter is so sensitive that they agreed to talk only on condition of anonymity.
Russia's ruling system, while projecting an image of unity, is divided along many lines -- between security hawks and economic liberals, between people with personal vendettas, and between competing business interests.
Putin holds the disparate interests together, so any hint of a vacuum at the center of the system is risky.
Putin is so entrenched within Russia's ruling system that many of its members can imagine no other leader. Many in state companies and major banks say they anticipate no real change at the top when Putin's next term ends.
"There are no discussions in the corridors about the succession. It's as if people know that he (Putin) will be around forever," said a source in a government ministry.
If Putin want the constitution changed to allow a third successive term, he will need two-thirds support in the lower house of parliament, three-quarters in the upper chamber and approval in two-thirds of regional legislatures.
All are institutions where Kremlin allies are the overwhelming majority but Putin has said will not change the constitution to stay in power.
If he did so, he would risk a backlash from voters who might see it as Russia turning its back on democracy. He also avoided the temptation to tinker with the constitution to extend his rule in 2008, when he last faced term limits.
Instead, he stepped aside and let a loyal lieutenant, Dmitry Medvedev, run for president, certain he would win with the Kremlin's backing. Putin, who became prime minister for four years, secure re-election when Medvedev's term ended in 2012, and Medvedev has been prime minister since then.
Putin controlled the country from the wings between 2008 and 2012, and might see a similar move as an option now. Medvedev's ratings are much lower than Putin's but the ruling elite could accept him as a tried-and-tested proxy for Putin.
"Only Medvedev," a source in government circles told Reuters when asked who could be president after Putin. "Everyone is afraid of change."
Age, though, might now get in the way for Putin.
He is in robust physical shape but will be 71 when his fourth term ends. If he again stepped aside for a full term, he would be 77 on his return -- the presidential term was extended to six years in 2008 during Medvedev's presidency.
One of the sources close to the Kremlin said Putin does sometimes feel extremely tired, but largely through exasperation at officials' incompetence and bureaucratic sloth.
During a meeting with Belarussian President Alexander Lukashenko in 2016, Putin was caught on an open microphone confiding to his ally: "I don't get enough sleep. The day before yesterday I slept for four hours, last night I got five hours."
Putin might also be loath to rule in his late seventies as he has portrayed himself as a healthy and energetic leader since replacing the ailing Boris Yeltsin and providing the vitality lacked by most leaders in the dying days of the Soviet Union.
He will soon have ruled longer than Soviet Communist leader Leonid Brezhnev, whose 18-year rule from 1964 to 1982 is primarily associated with stagnation, though dictator Josef Stalin ruled the Soviet Union for three decades.
To enjoy a restful retirement, Putin would need to anoint a fully fledged successor who can hold on to power in his or her own right and also protect the interests of the ruling elite.
Kremlin insiders say Putin has selected no heir apparent, and that any names being circulated are the product of speculation, not knowledge of Putin's thinking.
The names include Igor Sechin, head of state oil firm Rosneft; Defense Minister Sergei Shoigu; Igor Dyumin, a former Putin bodyguard who became a regional governor, and Moscow mayor Sergei Sobyanin.
Anointing a successor also entails dangers for Putin as doing so too soon would risk him becoming a lame duck, allegiances shifting from him to the heir apparent and new turf wars being set off in the ruling elite.
Above all, Putin will want to ensure any successor can hold on to power, will protect him and will not dismantle the system built around him.
Handing over to an heir, while eventually inevitable, will therefore also be Putin's riskiest path.
"The longer he stays in power, the harder it will be to exit," said Andrei Kolesnikov, senior fellow at the Carnegie Moscow Center, a think tank. "How can he abandon such a complicated system, which is essentially his personal project?"
(Additional reporting by Polina Nikolskaya, Denis Pinchuk, Darya Korsunskaya, Oksana Kobzeva and Polina Devitt, Writing by Christian Lowe, Editing by Timothy Heritage)