Party of Life, Party of Life, Rodina (Motherland), and the Party of Pensioners are holding separate congresses simultaneously at the Russian Academy of Public Administration, with the unification congress set for the afternoon.
The tri-party alliance is being formed to fight regional and parliamentary elections next year.
Sergei Mironov, who leads the Party of Life and is speaker of the Federation Council, earlier said the new party would have about 500,000 members, which would bring it close to Russia's traditional left-wing party, the Communist Party, and its 580,000 members.
"Time of small parties is over," he said,
He said the party could win the necessary 7% of the vote needed to enter the lower chamber of parliament, the State Duma, as it could take up a left of center niche where many voters' sympathies lie. He also suggested that it would be given access to airtime on the nationwide broadcasting channels.
Neither the Party of Life nor the Pensioners won enough votes in the 2003 election to take up seats in the State Duma. Rodina, which stormed into the Duma at the first attempt with just over 9% of the vote, has since been beset by problems.
Mironov said the new party would take up opposition to United Russia, the current "party of power," which holds a massive majority in the Duma.
