Rep. Bob Filner (D-Calif.) was expected to file his candidacy papers Wednesday to run for mayor of San Diego. A Democratic state Senator who has thrice challenged Filner is likely to run for and is favored to win Filner’s seat.

According to local reports, Filner has moved out of his Chula Vista home in the 51st district — outside of San Diego proper — and into an apartment complex in downtown San Diego. Filner is a former San Diego resident and elected official, having served on the school board for four years and city council for five years before being elected to Congress in 1992.

Filner will enter what is expected to be a crowded mayoral race. If no one receives 50 percent of the vote in the June 2012 open primary, the top two finishers will advance to a November runoff.

San Diego-based Democratic strategist Lucas O’Connor wrote on the Two Cathedrals blog that Filner “will need to re-introduce himself to both the people and the problems in San Diego” after 20 years of Congressional representation of only a small portion of the city.

Filner will leave behind a district President Barack Obama carried with 63 percent of the vote in 2008. It includes the entirety of California’s border with Mexico, though its exact shape in the next election will not be known until the districts are finalized in August.

Meanwhile, a Democratic source in San Diego said state Sen. Juan Vargas (D), described as Filner’s “longtime nemesis,” will be the favorite to win the seat. Vargas finished fourth in the 1992 Democratic primary for the open seat, then lost to Filner by 10 points in 1996 and 8 points in 2006.

According to the source, Vargas, considered a longtime moderate, has recently been shoring up support in the state Legislature among the more liberal factions of the party. A spokesperson from Vargas’ state Senate office did not return a call seeking comment on his future plans.