Boris Nemtsov, a deputy prime minister in the 1990s, said on Wednesday that, "There are two democratic opposition candidates in the presidential race, [former Prime Minister] Mikhail Kasyanov and me."
"My fellow party members and I believe there must be a sole democratic opposition contender," he added.
Nemtsov also condemned the use of administrative resources by the Kremlin to promote its candidate, First Deputy Prime Minister Dmitry Medvedev. He also called on Kasyanov and the other opposition contender, longtime Communist leader Gennady Zyuganov, to fight for equal campaign conditions.
"The legitimacy of the election is in their hands," he said.
Nemtsov had little chance of defeating Medvedev, whose candidacy has been endorsed by the highly-popular President Vladimir Putin. Opinion polls show a broad popular support for Putin's choice for his successor, and some Russian media have already begun calling Medvedev Russia's future president.
The SPS, which advocates a free market economy, performed well in the 1999 parliamentary election, but gained less than 1% of the vote at the December 2, 2007 polls.
The elections to the State Duma, Russia's lower house, were widely criticized as undemocratic, with the ruling United Russia party receiving over 90% of the vote in some regions. Chechnya, hit by two separatist wars in the 1990s and early part of this decade, reported that United Russia had won 99% of the vote.
Despite the withdrawal of Nemtsov and another opposition figure, Garry Kasparov, the ex-chess world champion, from the presidential campaign, Kasyanov himself has little hope of bettering anything that Nemtsov could have achieved.
Commonly known in Russia as "Misha two-percent," a reference to bribery allegations concerning his time in office, Kasyanov is heavily associated with the Yeltsin years, which saw widespread hardship in the country. Misha is the diminutive version of the Russian name Mikhail.