Rep. Chris Van Hollen, D-Md., talks at a news conference in Rockville on March 9 after announcing endorsements for his candidacy for U.S. Senate. (Brian Witte/AP)

Prince George’s County Executive Rushern L. Baker III on Wednesday will endorse U.S. Rep. Chris Van Hollen in the Democratic primary race to succeed retiring U.S. Sen. Barbara Mikulski, choosing the lawmaker from Montgomery County over his own constituent, U.S. Rep. Donna F. Edwards, who is also running.

Baker joins Montgomery County Executive Isiah Leggett (D) and the entire Montgomery County Council in backing Van Hollen, a seven-term congressman who served with Baker in the Maryland House of Delegates in the early 1990s.

Edwards, who entered politics as an outsider and has built a career on being politically independent, has drawn most of her support so far from national progressive groups. She has also been endorsed by Prince George’s County Council Chairman Mel Franklin.

“I’ve known Rushern Baker for over 20 years,” Van Hollen said in a statement released Tuesday night. “We stood side by side in the statehouse fighting for better schools and stronger gun laws, and we continue to fight hard for critical investments for Prince George’s County.”

Van Hollen and Edwards launched their candidacies within days of Mikulski’s saying she would step down after 2016.

Other prominent Democrats are openly considering entering the race, including Rep. Elijah E. Cummings (D), of Baltimore, who was in Annapolis on Monday to talk to state legislators about a possible run, and Rep. John Delaney (D), of Montgomery County.

Baker (D) said Van Hollen gives the Democrats the best chance to win the Senate seat. Endorsing early, Baker said, will help him build on the working relationship he already has with the congressman and could help Prince George’s shape the debate around the contest.

“My interest is in working with someone in the Senate who I know very well and who is going to be able to move up very quickly to work on important issues to us, like securing resources,” Baker said.

Baker said he worked with Van Hollen on legislation in Annapolis that brought additional funding to Prince George’s schools in the late 1990s. The borders changed due to redistricting.

“The things we love about Mikulski was that she is a fighter and she used her position to influence the outcomes of things,” Baker said. “So leadership and an ability to get things done plays a major role in my decision.”

Baker said he thinks that he is required, as county executive, to choose the person who is in the best position to take advantage of his or her office “to elevate the state and county.”

A win for Edwards — or for Cummings, if he runs — would mean the first black senator from Maryland in U.S. history. But Baker, an African American who leads the wealthiest majority-black jurisdiction in the country, said race did not factor into his decision.

“I think Donna is very smart and very talented and would do a terrific job,” he said. “But for me, I’m comfortable with Chris’s leadership because of our long-term working relationship.”

Baker will declare his support Wednesday morning at a event with Van Hollen and Leggett at the Colmar Manor Community Center in Prince George’s. The congressman helped lead the effort to find federal funding to build the center in 2010.