In Rockville, Montgomery County Council member Nancy Navarro speaks about her reasons for supporting the council's endorsement of Rep. Chris Van Hollen, center, for the U.S. Senate. (Doug Kapustin/For The Washington Post)

Democrats are bracing for a fractious primary fight for a rare open Senate seat in Maryland, with two members of Congress from neighboring jurisdictions ready to spar over their liberal credentials and records of helping working families.

Rep. Chris Van Hollen, a veteran lawmaker, scored a high-powered endorsement from Senate Minority Leader Harry M. Reid (D-Nev.) after jumping into the race last week, and on Monday, he won the backing of a slew of politicians in Montgomery County, where his congressional district is centered.

But that show of support has not dissuaded Rep. Donna F. Edwards, of Prince George’s County, who announced her candidacy Tuesday morning in a video message that she is e-mailing to supporters. She has already won promises of help from national progressive and women’s groups.

“We’ve worked with her and we know where her heart is on issue after issue,” said Eleanor Smeal, president of the Feminist Majority Foundation. “Donna has played a strong role in community activism for social justice . . . . We see this as a historic, breakthrough campaign.”

Edwards, a liberal hard-liner and Maryland’s first black congresswoman, would be only the fifth African American elected to the Senate. Some supporters say she would be a more fitting successor to retiring Sen. Barbara A. Mikulski than Van Hollen simply because she is a woman.

Rep. Donna Edwards (D-Md.) announces her intentions to run against Rep. Chris Van Hollen (D-Md.) for the U.S. Senate seat vacated by retiring Maryland Democrat, Sen. Barbara Mikulski. (Donna Edwards via YouTube)

But Van Hollen, a member of the House leadership, made clear Monday that he will not cede the progressive mantle to Edwards. At the same time, he said he intends to tout what he describes as his pragmatic approach.

“If you look at my record . . . it is one of being an effective progressive,” Van Hollen said. “Someone who has gotten things done.”

State politicians say it is possible that a more centrist Democrat from the Baltimore area — such as Rep. C.A. Dutch Ruppersberger — could also enter the race, hoping to appeal to moderate members of the party and those outside the Washington suburbs. Other Democrats are weighing bids as well.

The frenzy of interest, all within a week of Mikulski’s declaration that she would retire in 2016, could mean a debate over Democratic principles that is far more energetic than what has emerged so far among possible Democratic presidential hopefuls — a field that is overwhelmingly dominated by Hillary Rodham Clinton.

“They say Maryland is America in miniature,” said former Maryland governor Parris N. Glendening. “Right now, Maryland is playing the role of Democratic Party in miniature.”

Sen. Benjamin L. Cardin, who will become the state’s senior senator once Mikulski steps down, said a competitive primary will be “healthy for the party.”

But Craig Rice, a Democrat on the Montgomery County Council, said a battle over who is the most liberal could be harmful. “Trying to define who’s more progressive, I really don’t think that’s helpful,” said Rice, who endorsed Van Hollen and is the only African American on the council. “We saw that with the Republicans and the tea party.”

Some party stalwarts pointed out that Van Hollen, like Edwards, has enjoyed backing from liberal groups throughout his career. “He’s been a hero to progressives for a long time,” said Oscar Ramirez, a former vice-chair of the Maryland Democratic Party who recalled Van Hollen defeating Republican incumbent Connie Morella in a newly drawn 8th congressional district in 2002. “He’s been fighting our fight for a long time.”

But Valerie Ervin, a former Montgomery Board of Education and County Council member and a longtime friend of Edwards’s, described the Prince George’s Democrat as “the one that progressives turn to.”

“This is going to be a very difficult race for Democrats in Maryland. We have these two lions facing off,” said Ervin, who is also African American. She said the contest will raise important questions for party members. “Is the Democratic Party really democratic? Are the doors really open to embrace candidates who are not typical?”

Two officials with labor groups that are active in Maryland politics, who asked not to be named in order to discuss internal deliberations, said that both Edwards and Van Hollen are seen as strong union supporters.

“Nobody has labor locked up,” one official said. “There’s a very good chance we may not endorse.”

Bob Shrum, a Democratic strategist who helped Mikulski win in 1986, said he did not think the race would “be decided by some kind of ideological litmus test. . . . It’s going to be who connects with those folks.”

Republicans, meanwhile, are content to sit back and watch the battle play out.

There are more than twice as many registered Democrats as Republicans in Maryland, and most politicians and pundits think the Senate seat will remain in Democratic hands. But Republicans point out that anything is possible, as shown by the surprise victory in November of Gov. Larry Hogan (R).

“I don’t think he’s in any hurry to make a decision,” said Chris Meekins, a spokesman for Rep. Andy Harris, the only Republican member of Maryland’s congressional delegation and one of several GOP politicians who has expressed interest in the seat. “If Democrats have a bloody primary, it makes it much more in play.”

Del. Kathy Szeliga (R-Baltimore County), state House minority whip, said a bitter Democratic primary that pushes candidates to the left could open a “path to victory” for her party.

“I think it’s great,” she said of the Van Hollen-Edwards matchup, especially if the Democratic primary gets expensive. Republicans, she said, could “keep our primary less competitive and start raising money for the general.”

Rachel Weiner and Mike DeBonis contributed to this report.