Correction: An earlier version of this story incorrectly identified Bill Frick as a former state delegate; he is a current state delegate. The story also misidentified where Chris Van Hollen lives; he lives in Kensington, in Montgomery County.

Rep. Chris Van Hollen (D-Md.) speaks at an American Federation of Government Employees rally on Capitol Hill on Feb. 10. (Andrew Harnik/For The Washington Post)

Rep. Chris Van Hollen said Wednesday that he would run in 2016 for the seat being vacated by U.S. Sen. Barbara A. Mikulski, becoming the first Maryland politician to declare his candidacy in what is expected to be a crowded field.

The announcement, just two days after Mikulski said she would not seek a sixth term, could be an attempt to get out ahead of fellow Democratic Reps. Donna F. Edwards, Elijah E. Cummings, C.A. Dutch Ruppersberger, John Delaney and John Sarbanes — all of whom are mulling candidacies of their own.

“I am very grateful to the citizens of Maryland’s Eighth Congressional District for the opportunity to represent them,” Van Hollen, 56, said in an e-mail to supporters. “I am very much looking forward to the upcoming campaign and a healthy exchange of ideas.”

A formal announcement, he said, would come later in the week.

Van Hollen’s pending departure from the House will set off aftershocks inside the House Democratic Caucus. Now in his seventh term, he is a close ally of Minority Leader Nancy Pelosi (D-Calif.), who at 74 has previously considered retirement.

House Minority Whip Steny H. Hoyer (D-Md.), Pelosi’s No. 2 for 12 years, is known to want the top job. But Pelosi’s more-liberal allies have sought an alternative to Hoyer for several years.

With Van Hollen out of the picture, Hoyer’s path could become easier, unless the Democratic Caucus demands generational turnover and seeks a clean slate of leaders whenever Pelosi steps down.

Van Hollen, whose first foray into elected office was in the Maryland General Assembly, joined the House in 2003. The Montgomery County resident quickly became an authority on economic issues, rising to become the ranking Democrat on the House Budget Committee.

Aides say the congressman already has $1.7 million in his campaign coffers that he can use for his Senate run.

Van Hollen, like all current House members, would be up for reelection in 2016. Any member who attempts to replace Mikulski would have to give up his or her House seat. If several decide to run, that could create significant turnover among a group of lawmakers who have been in office for a cumulative 63 years.

The only member of Maryland’s congressional delegation who has not expressed at least some interest in the seat that Mikulski has held since 1987 is Hoyer, 75, who was elected to Congress in 1981.

Other Democrats said to be interested in the seat are Baltimore Mayor Stephanie Rawlings-Blake and former lieutenant governors Anthony G. Brown and Kathleen Kennedy Townsend. On the Republican side, Rep. Andy Harris, former governor Robert L. Ehrlich Jr. and former lieutenant governor Michael S. Steele are being discussed as possible contenders.

Edwards, who represents Prince George’s County and a slice of Anne Arundel County, is one of several women whose names are being floated. Edwards has drawn early support from progressive groups, but she said Tuesday that she wanted “a few days” to ponder whether to take the plunge.

“Edwards is more like Mikulski than any” of the other potential candidates, said friend and former state delegate Aisha Braveboy. A Senate bid would mean “leaving a district where she is beloved and going statewide. It’s a risk, but a risk she is willing to take.”

Kennedy Townsend said Wednesday that she, too, has made inquiries and is talking to friends — including Mikulski, the longest-serving woman in Congress.

“As the first woman lieutenant governor [in the state], I know the challenges women face, but I know there are also a lot of good candidates,” Kennedy Townsend said. Mikulski, she added, “is a groundbreaker for women, but she’s also a groundbreaker for working men and women. She has stood up for the little guy.”

Heather R. Mizeur, a former state delegate who unsuccessfully ran for governor last year, is also thinking about running and will probably make a decision within the next few weeks, said Joanna Belanger, her former campaign manager.

Van Hollen was immediately endorsed by Maryland Attorney General Brian E. Frosh (D), who served with him in the General Assembly, and Montgomery County Executive Isiah Leggett (D), who has known Van Hollen for years.

His departure from the House will launch a trickle-down scramble to represent the district, which meanders from Montgomery County north to Frederick and rural Carroll counties.

Among the Democratic names being mentioned are Montgomery County Council President George L. Leventhal (At Large); council members Hans Riemer (At Large), Craig Rice (Upcounty) and Roger Berliner (Potomac-Bethesda); state Del. Bill Frick; state Sen. Jamie B. Raskin; and former school board and council member Valerie Ervin.

Berliner, an energy lawyer with Hill staff experience and an ability to raise money, said he’s been approached and is “gratified” by the expressions of interest. But he said he could be more effective remaining on the council.

Paul Kane, Jenna Johnson and Bill Turque contributed to this report.