MOSCOW, December 17 (RIA Novosti) - The leader of the SPS party said on Monday he was resigning over his liberal opposition party's crushing defeat in the Russian parliamentary elections earlier this month.

The Union of Right Forces, known as SPS, secured 0.9% of the internationally criticized December 2 vote, which saw an overwhelming victory for the pro-Kremlin United Russia party. The threshold for the lower house of parliament was set at 7%.

"I bear full responsibility [for the defeat] and am stepping down," Nikita Belykh said at a party congress.

Belykh questioned the election results cited by election authorities, but admitted the party's and his personal failure to "forge a united democratic front" ahead of the polls.

He said all members of the party's governing council would also resign, and invited the congress to consider Boris Nemtsov, an ex-deputy prime minister and a former SPS leader, as the party's candidate in presidential elections on March 2, 2008.

But Anatoly Chubais, head of the electricity giant Unified Energy System and a member of the SPS governing council, said the party should not accept Belykh's resignation.

Chubais said it was a "courageous move," but it was "absolutely ill-timed." He said the party should review its strategy and objectives not to repeat previous mistakes.

He also doubted whether Nemtsov, a fierce advocate of Russia's privatization camapaign during the turbulent 1990s, would be successful as a presidential contender.

Other opposition candidates to challenge United Russia's nominee, First Deputy Prime Minister Dmitry Medvedev, in the presidential polls are Soviet dissident Vladimir Bukovsky and ex-Prime Minister Mikhail Kasyanov, who have already been formally put forward by their action groups.

Medvedev, who enjoys President Vladimir Putin's backing, is widely seen as a front runner.

Kommersant daily quoted Bukovsky on Monday as saying he had signed a deal with Kasyanov and Nemtsov to coordinate activities in the run-up to the elections.

All of them will attempt to collect the 2 million signatures required to support their application and then get registered with the Central Election Commission, but the one with the highest rating will run as a single opposition candidate, the paper said.