By Andrew Osborn

MOSCOW (Reuters) - Vladimir Putin appears politically invincible after Russia's ruling party won its biggest ever parliamentary majority this month. But he faces an increasingly pressing dilemma: How best to ensure the survival of a system built around himself.

With a presidential election due in March 2018, Putin, 63, must decide whether or not to run again. He must also decide whether to bring that vote forward to 2017 to reset the system early to hedge against the risk of a flat-lining economy.

Few outside his tiny coterie know what he will do. Most Kremlin-watchers are sure he will run again and win, delaying the successor question until 2024. Others say he may surprise.

On the face of it, staying on looks to be an obvious choice. Polls give Putin an approval rating of about 80 percent, the ruling United Russia party just won 76 percent of seats in parliament, and his annexation of Ukraine's Crimea sealed his savior-of-the-nation image in many Russian eyes.

But beneath the surface, Putin's problems are piling up. They include what is forecast to be an anemic economic recovery, the lack of an obvious successor, voter apathy, his own complaints about the physical demands of the job, and the risk of destabilizing clan infighting inside the system.

Increasingly, it also seems that the only way Moscow can reset ties with the West would be for Putin to stand aside. The United States and European Union imposed economic sanctions over Russia's actions in Ukraine in 2014 and thus far there has been little sign of a lifting of trade restrictions.

Nikolai Petrov, a political analyst at Moscow's Higher School of Economics, believes Putin could preserve the system's legitimacy if he handed over to a handpicked successor in 2018.

"It's a possible scenario," Petrov told Reuters.

He said he was skeptical Putin would choose that path, however, despite being under pressure to find alternative ways of maintaining broad support for the system beyond nationalism and foreign military adventures.

"Putin is a hostage of his own popularity," said Petrov.

People who know Putin say he is growing weary. In an unguarded moment picked up by microphones last year, he was heard complaining about how little he slept.

One former high-ranking official close to the Kremlin said Putin, in power either as president or prime minister for nearly 16 years, was fatigued.

"Putin is tired, he's getting older," the source, who declined to be named, told Reuters.

Dmitry Gudkov, a liberal opposition politician who lost his seat in this month's elections, told Reuters Putin looked certain to run again regardless because he was afraid stepping down might leave him vulnerable to prosecution for his actions in Ukraine.

"With a lot of enemies both inside and outside the country, he's starting to feel less secure. It doesn't look like a time when he'd give up control," said Gudkov.

Putin is fond of a surprise though. Many thought he would not step down from the presidency in 2008, but he did, albeit to make a triumphant return to the office four years later.

The source close to the Kremlin said the outcome of the U.S. presidential election and how the winner dealt with Russia initially was likely to influence Putin's decision.

"Putin is rather taken by global politics and won't run unless 'a firm hand' is needed," said the source. "Otherwise he will leave it to (Prime Minister Dmitry) Medvedev."

Democratic candidate Hillary Clinton would take a tough line with Moscow, unlike Republican contender Donald Trump who has said he wants to reset ties with Russia, people close to the Kremlin believe.

THE ENEMY WITHIN

The economic outlook is bleak. More than two years after the West imposed sanctions, their impact appears to be waning and the economy is expected to return to modest growth next year.

A continuing dearth of foreign investment, something that has played a major role in kick-starting growth in the past, means the recovery is likely to take years however and growth is forecast to reach only around 0.5 percent in 2017 and stay that way for a prolonged period.

Maintaining a semblance of popular support amid signs that growing numbers of voters believe their participation in elections is an empty ritual is becoming harder too.

Turnout at the Sept. 18 vote fell to a post-Soviet low.

And while there are no signs of serious unrest among the elite, Putin's allies are starting to worry that a threat might emerge from within the system one day.

"Our state is always destroyed from the top and from inside," Dmitry Olshansky, a columnist for the pro-Kremlin Komsomolskaya Pravda tabloid, wrote after the election, saying the appearance of the state's victory might be deceptive.

Such fears and the need to reshuffle officials to create the impression that the system is renewing itself help explain why Putin has replaced a slew of senior Kremlin, security and regional officials in recent months, a process seen continuing.

EARLY ELECTIONS?

Putin will have to make his mind up about the timing of the next presidential election soon.

Alexei Kudrin, an economics adviser to the government and a former finance minister, suggested bringing the vote forward to next year from 2018, saying that would allow the authorities to win a new mandate to launch tough reforms.

Kudrin, a Putin ally, did not say who he thought should stand, but the country's elite assumed he was talking about Putin.

The finance ministry fueled speculation that such a decision has already been taken, publishing a letter in July talking about a presidential vote in 2017.

The same source close to the Kremlin said there was now a more than 50 percent chance of an early presidential election.

Political analyst Petrov said he thought early elections were highly likely unless Trump won the U.S. presidency and lifted sanctions.

A different source close to the Kremlin said:

"By 2018, the economy won't be any better and the population will be weary. There will be more negativity around, Putin's rating will be falling, and our financial reserves will be running out," the source, who also declined to be named, said.

"All this backs the argument for early elections."

OPERATION SUCCESSOR

Even if, as is widely expected, Putin decides to run for president again, he will need to begin preparing a successor.

After years of fawning state TV coverage, many voters say they struggle to imagine political life without Putin.

"The president will find himself in a trap," the Carnegie Moscow Center said this month. "Legitimacy bestowed on a charismatic leader is not automatically passed down to his successors."

The only other politician regularly given prominence on state TV is Medvedev, the prime minister. He stood in as president from 2008-12 to help Putin skirt a constitutional ban on anyone serving more than two back-to-back presidential terms.

He is a potential successor, though many voters find his style too soft.

Speculation about other possible successors ranges from the defense minister to the governor of the central bank to the new and unknown head of the presidential administration.

One new name to have emerged after the elections is Vyacheslav Volodin, the former deputy head of the presidential administration. Putin has said Volodin should be the new speaker of parliament, a job that would give him a high public profile.

(Additional reporting by Elena Fabrichnaya, Katya Golubkova and Daria Korsunskaya; Editing by Janet McBride)