Lt. Gov John Garamendi is in the lead in a special general election Tuesday for an open congressional seat in the East Bay, according to unofficial election results.

With just over a quarter of the vote in, Garamendi, a Democrat, is leading by a large margin over Republican David Harmer in the race for California’s 10th Congressional District, which includes much of Contra Costa County, as well as parts of Alameda, Solano and Sacramento counties.

As of 8:30 p.m., Garamendi had received about 56 percent of the vote, compared with 40 percent for Harmer, with 29 percent of the district’s vote tallied by the secretary of state’s office.

“The returns are very good, but we’re not home yet,” Garamendi said. “But so far, it’s all good news.”

The election will fill the seat vacated by Democrat Ellen Tauscher, who was nominated in March for a State Department position that she accepted in June.

The winner of the race will serve the rest of Tauscher’s term, which ends in January 2011.

Jeremy Cloward of the Green Party, Mary C. McIlroy of the Peace and Freedom Party, and Jerry Denham of the American Independent Party have received the remaining 4 percent of the votes.

Garamendi had a built-in advantage heading into the election in the heavily Democratic district.

In September’s primary election, the six Democratic candidates received a total of 69,130 votes, compared to 37,040 for the six Republican candidates.