But Belykh looked likely to stay on after the party's top governing body proposed that the congress re-elect him. Delegates were to vote on Belykh's candidacy late Monday.
Earlier in the evening, the delegates overwhelming elected Boris Nemtsov as the party's candidate in March's presidential election.
"I understand the full scope of the responsibility I bear for the failure of the party at the parliamentary elections and I am resigning," Belykh said at the congress, held at the Hotel Izmailovo in eastern Moscow, Interfax reported.
Belykh, who has led the party since succeeding Nemtsov in 2005, said his "strategic mistake" was that the Union of Right Forces, or SPS, adopted a tougher stance against the Kremlin "too late."
SPS participated for the first time this fall in opposition protests, including a Dissenters' March organized by the Other Russia coalition last month.
The State Duma elections, in which SPS garnered less than 2 percent of the vote, plunged the pro-business party into more than $8 million of debt because only parties that crossed a 3 percent threshold qualified for free television airtime during the campaign.
SPS has one year to settle the debt, which threatens to severely handicap the party as it attempts to get Nemtsov on the ballot for the March 2 presidential vote. Nemtsov will have to collect 2 million signatures from voters, a huge number considering he only has around three weeks to do it. Jan. 16 is the Central Elections Commission's deadline.
But SPS and Nemtsov himself are placing more emphasis on promoting a single candidate from the oppositional parties than putting forward their own man, in hopes of not repeating the Duma failure. Nemtsov has signed an agreement with the two opposition candidates, former Soviet dissident Vladimir Bukovsky and former Prime Minister Mikhail Kasyanov, on pre-election cooperation, said Yelena Dikun, a spokeswoman for Kasyanov. She declined to say what that involved.
But Kommersant reported that the candidate with the highest public opinion rating after the signatures deadline on Jan. 16 would be left alone to run with the support of the other two.
Despite all the talk of a single candidate, nearly half of all voters doubt that the liberal parties will be able to agree on one, according to a survey released Monday by the Levada Center. One-quarter said the parties could. Bukovsky enjoys the highest rating of the three -- 5 percent -- after receiving Yabloko's backing on Friday, the survey said. Nemtsov comes next, with 3 percent, while Kasyanov is last, with 2 percent.
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