Rep. Chris Van Hollen (D-Md.) says he will run for the Senate seat that will be open when Barbara Mikulski retires next year. (J. Scott Applewhite/AP File Photo)

U.S. Rep. Chris Van Hollen, once considered the likely stand-in to House Minority leader Nancy Pelosi, has abandoned leadership prospects there for a chance to serve in the Senate, securing a major endorsement from that chamber’s leading Senate Democrat.

Senate Minority Leader Harry M. Reid (D-Nev.) announced Friday his support for the Maryland congressman to fill the seat that will be left vacant by Sen. Barbara Mikulski in 2016.

“Chris has also fought hard for greater economic fairness, including a tax code that rewards hard work and isn’t rigged in favor of special interests and the very wealthy,” Reid said in a statement. “I know he will be missed in the House... but I have no doubt that he will be an outspoken champion for democratic values in the United States Senate.”

Van Hollen is moving quickly to score support and donors ahead of what is likely to be a competitive Democratic primary for a rare open Senate seat few in Maryland politics can resist. The opportunity will likely push many of the congressman’s colleagues to launch campaigns of their own in the coming days and weeks.

In a radio interview Friday, the Montgomery County Democrat responded to reports that Pelosi had urged him not to run. The minority leader “wished me the best no matter what decision I made,” he said.

Van Hollen, who was first elected in 2002, declared his candidacy two days after Mikulski’s retirement announcement set off a whirlwind of speculation as to who would succeed her.

Although none have said so publicly, Democratic Reps. Elijah Cummings, John Sarbanes, C.A. Dutch Ruppersberger, John Delaney and Donna Edwards and Republican Rep. Andy Harris are said to be contemplating a Senate candidacy, among many others.

During the Politics Hour of the Kojo Nnamdi show Friday, Van Hollen defended his progressive record in what seemed like an effort to assuage liberals who may prefer the yet-undeclared Edwards. She is expected to announce a decision early next week, sources say.

Progressive groups are beginning to line up behind Maryland’s first black congresswoman after two liberal advocacy organizations launched a draft Thursday urging Edwards to run and began soliciting donations for her campaign.

On Friday, the super PAC that supports pro-choice women candidates Emily’s List — who first helped put Mikulski in office — promoted Edwards on social media without giving an official endorsement.

The race for Mikulski’s seat has attracted names both new, well-known and political.

Friends of former NAACP president Benjamin Jealous, a longtime state resident, told the Washington Post he is exploring a bid. Baltimore attorney Susan Burke, who has sued the Defense Department on behalf of military sexual assault victims, said she is “seriously considering” a run but has not made a decision.

Ex-state politicos such as former governor Robert L. Ehrlich Jr. (R), former lieutenant governors Anthony Brown (D), Michael Steele (R) and Kathleen Kennedy Townsend (D) and former state delegate Heather Mizeur (D) are among the talked-about potential contenders for the Senate seat, according to Maryland Democrats and Republicans.

In the radio segment, Van Hollen admonished voters to pay attention not only to a candidate’s agenda but also to their ability to “translate those values into action.”

“I believe in a progressive agenda, and I believe in being effective in getting that progressive agenda through,” Van Hollen said. He added that liberal groups advocating for women’s rights, the environment and worker rights have supported him in the past based on his “history of effectiveness and performance” as a lawmaker.

In an exchange with a Montgomery County caller to the show, Van Hollen was asked to clarify his position on entitlements. The woman prefaced her question by saying she would probably vote for Edwards: “I would like to know why [Van Hollen] would be better progressive candidate.”

Van Hollen responded saying he opposes any cuts to Social Security and Medicare and is “pleased to say very clearly that I support strengthening both these programs. I have a record of doing it and leading the fight in doing it.”

[Mikulski, a role model for generations of women in politics, to retire in 2016]

On Capitol Hill, Van Hollen emerged as a leader with a sharp economic mind on the House budget committee and one of the authors of the Democratic budget. He said he fought against the budget proposals of Rep. Paul Ryan (R-Wis.) that included changes to Medicare and Social Security.

Van Hollen said he has had a passion for the state since his days in the General Assembly and wants an opportunity to show Marylanders he can be the kind of effective leader Mikulski set as an example.

The congressman’s résumé and policy positions will continue to surface as Van Hollen pitches his candidacy statewide. The speculation about who would run for the open House seat in Maryland’s 8th Congressional district includes local council members and state legislators.

The latest is Montgomery County Council member Hans Riemer (D-At-Large), who has migrated from being mentioned as a candidate to publicly declaring interest in the race for Van Hollen’s seat.

In a Facebook post Thursday, Riemer said he loved serving on the council. But added: “When a chance to grab an oar and try to turn around the national government in a better direction comes around I have to take it seriously.”

Riemer, 42 and in his second council term, is a former senior adviser at AARP and was national youth vote director for the 2008 Obama campaign. In an interview Riemer, who lives in Takoma Park with his family, said he would take a couple of weeks consult with strategists to see if there was “a path to victory.”