PRIME Minister Vladimir Putin's party may win a reduced majority in Russia's parliament, forcing it to work with other political groups.
United Russia's backing fell to 50.1 per cent from 64 per cent in 2007, the Moscow-based Central Election Commission said on its website after 87.5 per cent of votes were counted.
"Given the new composition of the Duma . . . we will have to enter into coalition bloc agreements," said President Dmitry Medvedev to supporters at the party's campaign headquarters late Sunday Moscow time. "This is parliamentarianism. This is democracy."
The loss of United Russia's two-thirds majority, which allowed it to change the constitution single-handedly, is the first time the party's support fell from one nationwide election to the next since it was set up 10 years ago. Putin, 59, may be forced to make unpopular cuts in social spending and raise the pension age to balance the budget next year.
The prime minister, who plans to return as president next year to give him potentially almost a quarter-century in power if he runs again in 2018, lost support as stalling wage growth and the government's shortcomings in curbing corruption repelled voters.
"This is a personal defeat for Putin," said Dmitry Oreshkin, an independent political analyst based in Moscow. "He understands that his popularity is . . . falling increasingly fast."
Three other parties scored above the 7 per cent threshold for proportional representation in the legislative body. The Communists have 19.1 per cent of the votes counted so far, followed by the Just Russia party with 13 per cent. Nationalist Vladimir Zhirinovsky's Liberal Democratic Party, or LDPR, was at 11.7 per cent.
United Russia may still keep more than half of seats in the 450-member Duma, the lower house of parliament, Boris Gryzlov, the party's leader in the legislature, said.
Mr Medvedev may take over as leader of United Russia "within several months," Sergei Naryshkin, head of the presidential administration, told state-owned Vesti television. Half of the United Russia deputies in the new parliament did not serve in the previous one, he said.
"If United Russia fails to get a majority, then the government will have to rely on support from the LDPR to get legislation through," Chris Weafer, Moscow-based chief strategist at Troika Dialog, Russia's oldest investment bank, said. "This would slow, if not prevent, any unpopular legislation, including cuts to social spending or changes to the retirement age."
Speaking after the President, Mr Putin said voters "maintained" United Russia's "role as the leading political party.
"It's the optimal result. We can guarantee the stable development of our state."
He warned last week against "smashing" the parliament's unity and suffering the political paralysis afflicting Europe and the US as Russia seeks to avoid contagion from the euro region's debt crisis.
United Russia's "results are obviously lower than expected, so the market could potentially interpret this as slightly negative," Ivan Tchakarov, chief economist at Renaissance Capital in Moscow, said.