No special election has been scheduled. There isn’t even a vacancy.

Yet the race to replace Rep. Ellen Tauscher (D-Calif.) switched into high gear Wednesday, with the addition of two top-tier candidates.

Lt. Gov. John Garamendi (D) announced that he was abandoning his 2010 gubernatorial campaign to run for Tauscher’s seat if it becomes vacant.

“I am excited by this unexpected potential opportunity in my own backyard, and with your support, I will return to Washington to help shape a better future for California, our country and a very special Congressional district,— Garamendi said in a statement.

Meanwhile, state Assemblywoman Joan Buchanan (D) is now all but certain to join the race as well, according to sources in California and Washington, D.C.

Even though a few other candidates have already entered the race and more could follow, Garamendi and Buchanan join state Sen. Mark DeSaulnier (D) as the early frontrunners in a contest that could still be several months off.

“There are really only three Democrats who are viable,— said Chuck Carpenter, chairman of the Contra Costa County Democratic Party, where the bulk of the 10th district’s voters are. And while candidates from all political parties will appear together on the special election primary ballot, Democrats are heavily favored to hold the East Bay seat.

President Barack Obama nominated Tauscher last month to be undersecretary of State for arms control and international security. But while her confirmation is all but certain, no hearings have been set on her nomination, so the candidates find themselves in the awkward position of campaigning for a job that isn’t available yet.

That’s why Garamendi, in announcing his candidacy with visits to two public-private East Bay employment centers Wednesday, said only that he “expects— to seek the seat.

But any uncertainty over the timetable of the special election hasn’t prevented the candidates from moving quickly. In fact, within hours of Tauscher’s nomination, Assemblyman Tom Torlakson (D), who many East Bay Democratic insiders expected to be the frontrunner for the seat, said he was looking seriously at running, only to change his mind 24 hours later and say he intended to stay in the 2010 race for California superintendent of public instruction.

“He could have walked into this seat,— Carpenter said.

Instead, DeSaulnier, a close ally of Torlakson’s, jumped into the Congressional race, and for several weeks he looked like the leading contender. Tauscher and Rep. George Miller (D), who represents an adjoining East Bay district, promptly endorsed him, and more recently, key labor organizations such as the Contra Costa County Central Labor Council have gotten behind him as well.

But Garamendi’s decision to abandon the gubernatorial contest, where most polls on the Democratic primary showed him placing fourth, scrambles the 10th district political dynamic in myriad ways, and may force several party activists to rethink their allegiances.

Garamendi has been a fixture in state and local politics for decades. A rancher who lives on the eastern edge of the Congressional district, he is nevertheless well-known and well-liked among Democratic voters in the area.

He was first elected to the state Assembly in 1974, won a state Senate seat two years later, and in 1990 was elected as California’s first state insurance commissioner. In 1995, President Bill Clinton named Garamendi deputy secretary of the Interior, and in 2002 he won his old post as insurance commissioner. Four years later, he became lieutenant governor.

In 1992, Garamendi’s wife, Patti, was the Democratic nominee in a contentious open-seat Congressional race with Republican rancher Richard Pombo, who won by 2 points and served for 14 often-controversial years. The 11th district as it was drawn then included part of what is now in Tauscher’s district, and Patti Garamendi spent more than $800,000 on the race.

On May 9, the family will hold its 32nd annual Basque Barbecue at its Touch the Earth ranch in Paloma. Originally intended as a fundraiser for Garamendi’s gubernatorial campaign, the barbecue may now be used to jump-start his Congressional committee.

Al Pross, an editor at the California Target Book, a political tip sheet, said that it was “wise for DeSaulnier to get in the race early— and start racking up endorsements. But he said Garamendi’s candidacy presents a dilemma for 10th district Democrats, and added that it is not clear whether the endorsements now will translate into huge waves of grass-roots support.

“I expect there will be numerous double endorsements,— Pross said.

But Carpenter predicted that DeSaulnier’s support from the Members and unions will be persuasive, and he said that even though his county party will remain neutral in the special election, he will back the state Senator. DeSaulnier is also a veteran officeholder, having served in both chambers of the Legislature, on the Contra Costa County Board of Supervisors and on the Concord City Council.

“As chair of the party, my feeling is it’s Mark’s time to do this,— Carpenter said. “Both Congresswoman Tauscher and Congressman Miller have already endorsed him, so who am I to argue with the leaders of my party in my county?—

But the plot thickens: Some Democratic activists are encouraging Garamendi to forgo the 10th district race and run instead in the nearby 3rd district, where Rep. Dan Lungren (R) barely cleared 50 percent in 2008 in a district that had been considered a GOP stronghold. Those assumptions were smashed when Obama and Sen. John McCain (R-Ariz.) each got 49 percent in the 3rd district in the presidential election; George W. Bush had won a 17-point victory there four years earlier.

Amid rumors that Lungren’s 2008 opponent, Bill Durston (D), doesn’t plan to run again, there is already a draftgaramendi.com Web site, as well as Facebook and Twitter pages urging the lieutenant governor to challenge Lungren. Andrew Stone, a spokesman for the Democratic Congressional Campaign Committee, said D.C. Democrats are aware of the movement to draft Garamendi into the 3rd district race, but would not comment further, except to say, “That district is one that we’re really paying close attention to.—

One consequence of Garamendi’s candidacy in the Congressional race is that it is all but certain to push Buchanan into the race as well. Buchanan has been pondering a Congressional bid for weeks — she even commissioned a poll in March showing her with a slim lead over DeSaulnier, though with many voters undecided. But Democrats on both coasts said Buchanan, who was just elected to the Legislature in an expensive race last year, capturing a seat that previously had been held by a Republican, believes her chances in a special election improve with two strong male candidates also running.

“The one thing you can say about the East Bay is it has shown a more than favorable attitude toward female candidates,— Pross said. “But on the other hand, on the head to head, unless she could raise a whopping amount of money, I wouldn’t expect that she’d be ready.—

However, Buchanan, who spent two decades on the San Ramon Valley School Board until last year, could have a ready source of cash if she is endorsed by EMILY’s List, which backed her during her competitive Assembly race last year.

“We know she’s a terrific candidate, and we’re watching this race with interest to see how it develops,— said Jonathan Parker, the group’s political director.

Sources said EMILY’s List, which supports Democratic women who favor abortion rights, will consider Tauscher’s endorsement of DeSaulnier and the fact that he has strong grass-roots support in the district as it considers whether to become involved in the special election. And some Democrats suggested that Buchanan’s tough victory in last year’s legislative race could hamper her Congressional campaign, inasmuch as party leaders would be worried about holding the legislative seat.

Meanwhile, other candidates have jumped into the race, including attorney Adriel Hampton (D), Fairfield City Councilwoman Catherine Moy (R) and businessman Chris Bunch (R). Republicans believe they may have stronger potential candidates in Contra Costa County Sheriff Warren Rupf, who wins his countywide office in nonpartisan elections, and former San Francisco 49ers player Brent Jones.

“Given the strong Republican talent in this district, we are confident that we will have a viable, fiscally responsible candidate who can offer voters a striking contrast to their other detrimental option — yet another Democrat who will act as a rubber stamp for Nancy Pelosi and her irresponsible spending spree,— National Republican Congressional Committee spokeswoman Joanna Burgos said.