WASHINGTON - Rep. Ben Cardin, D-Baltimore, is expected to announce he will run for the Senate seat being vacated by Sen. Paul Sarbanes in 2006, setting up a "good, competitive" Democratic primary that could see more candidates yet.

Cardin refused to confirm his plans Thursday, saying he will announce his intentions "shortly." Aides said that announcement could come next week.

But sources said the 10-term House member will definitely run for Senate, setting up a primary race against Kweisi Mfume, a former Democratic congressman and recent president of the National Association for the Advancement of Colored People.

Democratic Reps. Chris Van Hollen of Kensington and Elijah Cummings of Baltimore have also expressed interest in the seat, but have not yet said they will run.

Rep. Dutch Ruppersberger of Cockeysville, one of the few Democrats who has taken his name out of the running, said Cardin would be a good addition to the race if he does run.

"He has really got a tremendous amount of knowledge about issues on the Hill, and he knows very much about the issues that are important to Maryland," Ruppersberger said.

Barbara Hoffman, a former Maryland state senator, said it would not be surprising if Cardin did decide to run for Senate, noting he "has been in the House for a long time, this is an opportunity for him to take on a much larger role."

A race between Mfume and Cardin would be a close one, said Keith Haller, the president of Potomac Inc., a research firm that did a statewide poll of likely voters last week for The (Baltimore) Sun.

"In a two-way race between Kweisi and Cardin -- it is hard to predict, but the race would be within the margin of error," Haller said. When Van Hollen is added to the mix, Mfume takes the lead "but the other two are not that far behind," Haller said.

But of the three Democrats, Cardin would do best in a Senate race against Lt. Gov. Michael Steele, edging out the Republican, according to Haller's poll.

"Cardin has accrued strong support in the Baltimore region -- which allows him to penetrate the Republican base," Haller said.

Cardin, a former speaker of the Maryland House of Delegates, is often described in the same terms as Sarbanes: an intellectual legislator who is not too flashy.

Mfume calls him a friend, but said Maryland would "lose ground" if Cardin were to step down from the committee posts he has earned during his years in the House.

"If Ben does make that decision (to run), Maryland will lose its top-ranking position on both the trade and human resources subcommittees of the most powerful committee in Congress -- which is the Ways and Means Committee," Mfume said.

He said Cardin is needed on the committee as it takes up Social Security reform and wrestles with the trade deficit.

"My hope is that reconsiders running against me and continues to serve . . . the state of Maryland in the very important post he is in," Mfume said.

Van Hollen, who has set up an "exploratory team" to weigh a Senate bid, said a Cardin candidacy would not change his plans.

"I'm in the same position I've been in," Van Hollen said. "I am talking to the people from our state and trying to determine how I can best serve the people of Maryland at this time."

He said he was not concerned by the growing Democratic field.

"A good competitive primary is healthy as long as people stick to the issues and focus on the goal of making sure that we ultimately have a candidate who reflects the values and principles of the people of Maryland," Van Hollen said.

Other Democrats also welcomed the competition. Maryland Democratic Party spokesmen Derek Walker said a spirited primary could only serve to "elevate the party, the activists, the volunteers."

Hoffman agreed.

"It is a good time to be a Democrat -- there is a wealth of good candidates," she said.