By Conor Humphries

MOSCOW (Reuters) - Russians voted on Sunday in regional elections that tested the strength of Prime Minister Vladimir Putin's ruling party less than 18 months before a presidential poll that could return him to the Kremlin.

The United Russia party, shaken by the acrimonious dismissal of founding member Yuri Luzhkov as Moscow mayor last month, is expected to dominate seven regional and thousands of municipal elections in which over 40 million people were eligible to vote.

But analysts said even isolated losses could prod the Kremlin to reconsider its dependence on the party to deliver an overwhelming victory in the March 2012 presidential election for either Putin or his protege, President Dmitry Medvedev.

"If the results are bad, the preparations for 2012 will need to change fundamentally," said Nikolai Petrov of the Carnegie Moscow Centre think tank.

"They may yet scrap the idea of a single dominant party altogether," he said, suggesting Russia's leaders might bolster a rival party to ensure the broadest possible support in elections they hope will preserve their power for years to come.

Putin and Medvedev have hinted one of them, but not both, will run as the Kremlin candidate for a six-year-term in 2012.

An unconvincing showing on Sunday could also prompt the authorities to boost public spending, which is already expected to rise in the year before the elections, said Chris Weafer, chief strategist at investment bank Uralsib.

Two smaller pro-Kremlin parties and the Communists will try to boost their shares of seats in seven provincial legislatures after a relatively strong showing in regional polls last spring.

In the violence-plagued North Caucasus, a village administration chief was fatally shot in a brawl that erupted after 4,600 ballots for a district election went missing, Russian news agencies reported, citing police.

In the industrial Chelyabinsk region in the Ural Mountains, entrepreneur Igor Frishko, 46, said he opposes the entrenched ruling party but would not vote for a provincial legislature.

"My vote won't change anything in the region or the country," said Frishko. "If I did vote, I would do so not out of any political predilection but in defiance of United Russia."

Chelyabinsk pensioner Vladimir Afanasyev, 62, said he would vote for United Russia "because this party has power and money."

LIBERALS EXCLUDED

Russia's main pro-Western liberal party, Yabloko, only managed to register in one of the seven regional votes. Other groups critical of the Kremlin say they have been blocked from registering as parties, barring them from elections.

Medvedev has promised democratic advancements, but liberal Kremlin opponents say there have been no significant changes.

"We had high hopes when Medvedev came to power, but it hasn't just failed to get better, it has gotten worse," said Grigory Melkonyan, deputy head of Golos, Russia's leading independent vote watchdog.

Indirectly, the vote was the first electoral test of Putin's popularity since wildfires swept Russia this summer during the worst heatwave since records began, killing more than 50 people and ruining crops over an area the size of Portugal.

United Russia's more than 2 million members include much of the elite in the nation of 142 million, united under a vague ideology that combines patriotism with support for Putin.

But after a decade in which the party has dominated politics from the national parliament down to local governments, there are some signs of fatigue among Russians.

March elections showed a decline in support for United Russia in most regions from 2007 federal elections. The party blocked the reappointment of the Kaliningrad region governor this summer after 10,000 people demanded his resignation.

Divisions were exposed by a dispute that ended with Medvedev's dismissal last month of longtime Moscow mayor and United Russia ruling council member Yuri Luzhkov.

(Additional reporting by Natalia Shurmina; editing by Tim Pearce)

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