The parties are holding separate congresses simultaneously at the Russian Academy of Public Administration, with the unification congress set for the afternoon.
Sergei Mironov, who leads the Party of Life and is speaker of the Federation Council, said the party would unite the left-leaning Party of Life, Rodina (Motherland), and the Party of Pensioners to fight regional and parliamentary elections next year.
Mironov earlier said the new tri-party alliance 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,
Neither the Party of Life nor the Pensioners won enough votes in the 2003 election to take up seats in the lower chamber of parliament, the State Duma. The Communists won 12.6% of the vote.
Although Rodina, running as an electoral bloc with a campaign focusing on the disparity between many ordinary people and tycoons, stormed into the Duma at the first attempt with just over 9% of the vote, it has since been beset by problems. Leaders have come and gone, and it courted controversy with a television advertisement in the campaign for the Moscow legislature this year that was called overtly racist by many and was eventually banned.
