U.S. Rep. Donna Edwards (D-Md.) will announce on Tuesday that she intends to run for the Senate seat being vacated by Barbara A. Mikulski (D). (Linda Davidson/The Washington Post)

U.S. Rep. Donna F. Edwards (D-Md.) plans to announce on Tuesday that she will run for the Senate seat being vacated by Barbara A. Mikulski (D), according to two Democrats familiar with her plans, setting up a potentially bruising primary fight with Rep. Chris Van Hollen (D-Md.).

Van Hollen — who had been considered a possible successor to House Minority Leader Nancy Pelosi (D-Calif.) — declared his intention to run for Senate last week and has already secured the endorsement of Senate Minority Leader Harry Reid (D-Nev.).

Edwards, 56, has drawn early support from national progressive organizations, who helped her oust incumbent Albert Wynn in the 2008 Democratic primary and last week announced a movement to get her to run for Senate. But it remains to be seen whether efforts by groups such as Emily’s List and the Feminist Majority will be enough to offset an early funding advantage held by Van Hollen, a proven fundraiser who has $1.7 million in his campaign account.

Edwards’s decision to run was confirmed by two Maryland Democrats with whom she has spoken about her plans. They asked for anonymity in order to speak candidly about what she had told them. Edwards spokesman Benjamin Gerdes said Edwards “is seriously considering a run for the United States Senate and will make a decision in the coming days.”

Mikulski’s decision to retire after 2016 has triggered a scramble among Maryland politicians eager to capi­tal­ize on a rare opportunity. U.S. Reps. Elijah E. Cummings, C.A. Dutch Ruppersberger, John Delaney and John Sarbanes — all Democrats — have also indicated some interest in the seat, along with Republican Rep. Andy Harris. Depending on how many jump in, the race could create significant turnover in Maryland’s Congressional delegation.

Reid’s quick endorsement of Van Hollen was likely an attempt to stave off a bitter intra-party fight — but it appears not to have worked.

Van Hollen, whose district includes voter-rich Montgomery County, is quickly lining up donors and endorsements, including from Maryland Attorney General Brian E. Frosh and Montgomery County executive Isiah Leggett. Additional county leaders will announce their support at an event in Rockville on Monday, according to his campaign.

As Maryland’s first black congresswoman, Edwards could appeal to African American and female voters, both strong Democratic constituencies. She also could try to stake a claim to be well-suited to succeed Mikulski, the longest-serving woman senator. Edwards, who is in her fourth term, represents a district that includes Prince George’s and parts of Anne Arundel counties.

A hard-fought primary could create unexpected openings for Republicans in the state, who lag far behind Democrats in voter registration but were buoyed by the victory of Gov. Larry Hogan (R) in November.

“I think, in this environment, we will have an opportunity,” said Joe Cluster, executive director of the Maryland Republican Party. He said he expects the primary race to grow to a party of three, with a Baltimore-area Democrat entering the contest soon.

Cluster said he does not expect any Republican to commit to a Senate bid until it is clear that the competitor will be a more-liberal Democrat from which to draw clear distinctions. If Ruppersberger, a conservative Baltimore-area Democrat, ran and secured the nomination, that would make a challenge from a Republican more difficult.

Cluster said he is receiving more interest from Republicans in the congressional races than for the Senate.

He described both Van Hollen and Edwards as “pretty liberal, and I don’t see them offering a whole lot of difference.

“But it will be interesting to see an African American woman versus a white man. . . . She is the only declared female and minority, so that could play an interesting role in the primary.”