In a move that caught his Senate colleagues off-guard and has earth-shattering political implications for his home state, Sen. Paul Sarbanes (D-Md.) announced this afternoon that he will not seek a sixth term in 2006.

“It was not my ambition to stay there until they carried me out,” Sarbanes said during a news conference at his district office in downtown Baltimore, according to The Associated Press. “It was just the right time. We think we’ve served long and well and honorably and we’re very comfortable with this decision.”

Sarbanes, the longest-serving Senator in Maryland history, becomes the third Member of the chamber to declare his intention to retire this cycle, joining Majority Leader Bill Frist (R-Tenn.) and Sen. Mark Dayton (D-Minn.).

Senate Democratic leaders had been confident of Sarbanes’ ability to win another term and were urging him to run again. Now, his retirement will likely set up a wild and crowded Democratic primary to replace him, and it gives Republicans at least a glimmer of hope in a heavily Democratic state.

Former NAACP President Kweisi Mfume, who spent nine years representing Baltimore in the House, may announce his intention to enter the Democratic primary as early as today. And five of the state’s six House Democrats are seen as at least potential 2006 Senate candidates: Reps. Benjamin Cardin, Elijah Cummings, Dutch Ruppersberger, Chris Van Hollen and Albert Wynn.

Only House Minority Whip Steny Hoyer (D) appears to be not eyeing the Senate race at this point.

“While the Senator’s leadership, intellect and courage will be sorely missed, Maryland is fortunate to have a wealth of tremendous Democratic candidates who are enormously qualified to continue Sen. Sarbanes’ legacy,” said Terry Lierman, chairman of the Maryland Democratic Party.

Within minutes of Sarbanes’ news conference, Cardin and Van Hollen released statements saying they would consider the race. And Wynn is reportedly going to establish an exploratory committee for a Senate race early next week.

Ruppersberger, in an interview, said he would also establish an exploratory committee.

“When something like this happens you have to move quickly and make sure you’re viable,” he said.

Some Maryland Democrats hold out the hope that one of their two candidates for governor in 2006 — Baltimore Mayor Martin O’Malley and Montgomery County Executive Douglas Duncan — will pivot and choose to run for Senate instead, avoiding a costly and potentially bloody primary as they try to take back the governor’s mansion. But both men seemed determined to stay in the gubernatorial race, as the Democrats attempt to unseat rookie Bob Ehrlich, the first Republican governor elected in the Free State since 1966.

Prince George’s County States Attorney Glenn Ivey (D), a former Sarbanes aide who is personally close to the Senator, is also mentioned as a possible Senate candidate. He is also seen as a leading contender for Wynn’s seat if Wynn runs for the Senate.

National Republicans would likely try to pressure Lt. Gov. Michael Steele into the race.

But John Kane, chairman of the Maryland GOP, said Steele prefers to seek re-election in 2006, with an eye toward running for governor in 2010.

State Sen. E.J. Pipkin, the 2004 Republican Senate nominee who took 34 percent of the vote against Sen. Barbara Mikulski (D), told the AP that he would decide on next year’s Senate race after the state legislative session ends in April.

Kane, a wealthy businessman who is occasionally mentioned as a possible candidate for statewide office, said he would not run. But he said the GOP is optimistic about its prospects in an open-seat Senate race, especially if Democrats “put up a real liberal, if they put up the wrong candidate.”

Many Maryland Democrats believe that a Senate vacancy — and the prospect of several House Members seeking to move up — will create so much activity down the ballot and interest in state politics that their prospects for defeating Ehrlich will increase.

“As a result of having so many potential candidates, when the ocean rises, all ships rise with it,” Lierman said.

But Patrick Gonzales, an independent pollster in Annapolis, said that scenario presents some peril for Democrats as well.

“Sometimes interesting can be bad, so you have to be careful,” he said. “The potential now is for a bruising primary for governor and Senator.”

Erin P. Billings contributed to this report.