In a move that caught his Senate colleagues off-guard and is causing major aftershocks in his home state, Sen. Paul Sarbanes (D-Md.) announced Friday that he will not seek a sixth term in 2006.

Five of the state’s six Democratic House Members said Friday they are considering running for Sarbanes’ seat. Former NAACP President Kweisi Mfume is also a likely Democratic candidate.

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

“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.

Some Democratic leaders in the Free State cast the 72-year-old’s decision as a selfless act. Nervous Democrats faced the prospect of having white male senior citizens running as incumbents for three of the four statewide offices on the ballot in 2006, potentially dampening the enthusiasm of party activists and voters as the party attempts to knock off freshman Gov. Bob Ehrlich (R).

“The Sarbanes I know spent a lot of time thinking about this whole thing. … He wants what’s best for the Democratic Party,” said Rep. Elijah Cummings (D-Md.), one of the House Members thinking of running for Senate.

Sarbanes’ departure loosens the longstanding political logjam in the state — its last Senate vacancy was in 1986 — and with several House Members eyeing the Senate race, it could create excitement up and down the ticket.

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

Still, colleagues in the state and in the Senate were lamenting the departure of a low-key but respected legislative veteran who made his mark with the Sarbanes-Oxley corporate reform bill and was a quiet, reflective liberal.

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 — a development that gives Republicans at least a glimmer of hope in a predominantly Democratic state.

Sarbanes’ announcement “provides Republicans a wonderful opportunity to gain another seat in the United States Senate,” National Republican Senatorial Committee Chairwoman Elizabeth Dole (N.C.) said in a statement.

But Democrats remain confident that they will hold the seat. Last fall, the state gave Sen. John Kerry (Mass.), the Democratic presidential nominee, 56 percent of the vote.

“Make no mistake about it: Maryland is a blue state and will stay that way in 2006,” said Democratic Senatorial Campaign Committee Chairman Charles Schumer (N.Y.).

Mfume, who spent nine years representing Baltimore in the House, issued a statement Friday praising the Senator but making no mention of his own political plans. Sources said he is expected to make an announcement today.

In addition to Cummings, Democratic Reps. Benjamin Cardin, Dutch Ruppersberger, Chris Van Hollen and Albert Wynn all said Friday that they were considering the race. In an interview, Ruppersberger said he would establish a Senate 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 said they were determined to stay in the gubernatorial race as the Democrats train their firepower on Ehrlich, the first Republican governor elected in the Free State since 1966.

According to a source close to O’Malley, the mayor’s Washington, D.C.-based political consultants were calling national Democratic leaders Friday in an attempt to deflect any pressure on O’Malley to run for Senate.

Prince George’s County States Attorney Glenn Ivey (D), a former Sarbanes aide, is also mentioned as a possible Senate candidate. Ivey, who is also seen as a leading contender for Wynn’s seat if Wynn runs for the Senate, said Friday it was too premature to talk about his political plans.

There would be no clear frontrunner in a wide-open Senate primary, and issues of race, geography, ideology and fundraising ability could prove decisive. Cummings, Ivey, Mfume and Wynn are all black; the other potential Democratic candidates are white. Ivey, Van Hollen and Wynn have bases in the Washington, D.C., suburbs, while the others are strong in the Baltimore region. Most of the Democrats are reliable liberals, with Ruppersberger the closest to the center.

At the end of 2004, Van Hollen was the leader among the House Members in cash on hand, with $471,000. Wynn was next with $274,000, followed by Cardin ($176,000), Ruppersberger ($118,000) and Cummings ($72,000). Mfume, who has an active federal campaign account from his days in the House, had $98,000 in the bank at the end of the year.

An independent poll conducted early this year about the possibility of an open Senate primary in 2006 found O’Malley, Mfume and Cardin bunched together.

The poll of 402 likely Democratic voters, conducted Jan. 3-8 by Gonzales Research & Marketing Strategies of Annapolis, showed O’Malley with 26 percent of the vote, Mfume favored by 24 percent, and Cardin the choice of 20 percent. The remainder of those questioned were undecided. The poll, which had a 5 percent error margin, did not ask voters about other potential candidates.

Pollster Patrick Gonzales said the wide-open scenario presents some peril for Democrats.

“The potential now is for a bruising primary for governor and Senator,” he said.

But it is difficult to see a strong Republican contender on the horizon. Party leaders in Washington would likely try to lure Lt. Gov. Michael Steele, who has gained a national profile as one of the few black Republicans in statewide office, into the race.

But John Kane, chairman of the Maryland GOP, said Friday that Steele prefers to seek re-election in 2006, with an eye toward running for governor in 2010. Rep. Wayne Gilchrest (R), has also taken himself out of consideration.

State Sen. E.J. Pipkin (R), the 2004 Senate nominee, will decide on next year’s Senate race in April.

Erin P. Billings contributed to this report.