Senator Barbara A. Mikulski, a former social worker from Baltimore who became a leading liberal voice in Washington and the longest-serving woman in Congress, announced in an emotional press conference on Monday that she would not seek a sixth term in 2016.
Mikulski, who rose to become the top Democrat on the Senate Appropriations Committee and the dean of Maryland's congressional delegation, told reporters gathered in Fells Point that she wanted to spend the next two years helping to groom the next generation of Democratic leaders in the state.
"Do I spend my time raising money or raising hell to meet your day-to-day needs? Do I spend time focusing on my election or the next generation?" Mikulski said. "The more I thought about it, the more the answer became really clear."
The announcement came as a shock to many Democrats — particularly those in Maryland — who thought Mikulski would remain in the post for another term in hopes that Democrats would reclaim the Senate in a presidential election year.
Polls have widely shown Mikulski to be the most popular elected official in the state.
"Senator Mikulski is more than just a legendary senator for the people of Maryland, she's an institution in the United States Senate," President Barack Obama said in a statement. "Barbara's service to the people of Maryland spans decades, but her legacy will span generations."
The decision set off a flurry of speculation about her possible replacement in a state with a small, but powerful congressional delegation. Several leading names were floated, including former Gov. Martin O'Malley, who has been considering a run for president but who has performed poorly in polls compared with Hillary Clinton.
O'Malley's former lieutenant governor, Anthony Brown, is seriously considering the seat, according to aides who spoke on the condition that they not be named. Rep. John Delaney, a Western Maryland Democrat, announced on Twitter that he, too, is considering a run for Senate.
"There could be a huge number of Democrats in the primary," said Don Norris, director of the school of public policy at UMBC.
Mikulski declined to say whether there were any particular potential candidates she thought might be good for the job.
"Maryland has a lot of talent," she quipped. "They'll be telling you about it in the next 10 minutes."
And although the state has historically elected Democrats statewide — particularly in federal races and especially in a presidential election year — a handful of Republicans may also look at the seat, including former Gov. Robert L. Ehrlich. Dan Bongino, who secured the Republican nomination for Senate in Maryland in 2012, said he is also weighing whether to run.
"After winning the governor's race in 2014, there's no question that an open Senate seat in Maryland instantly becomes a top pickup opportunity for Republicans," said Andrea Bozek, a spokeswoman for the campaign arm of Senate Republicans. "While Democrats get ready for a bloody primary, we will have a top recruit waiting for whoever emerges."
Mikulski, often described as "tough as nails," became emotional as she recalled her years growing up in Baltimore and thanked Maryland voters for honoring her "with your confidence and trust."
Mikulski, 78, is a Highlandtown native and Maryland's senior senator. A former member of the Baltimore city council, she was first elected in Washington in 1976 to the House of Representatives. She has served in the Senate since 1987.
"It's going to be a donnybrook," Senate President Thomas V. Mike Miller said of the campaign for Mikulski's replacement.
"It creates turmoil down the entire chain. I've had three would-be congressmen call me already and tell me not to make any decisions. … There is no unity. There is no party boss. There is no party discipline. It's a free-for-all."
Senator Jon Tester, Chair of the Democratic Senatorial Campaign Committee, said he is confident another Democrat would emerge "and make Barbara Mikulski proud."
Baltimore Sun reporters Yvonne Wenger, Erin Cox, Liz Bowie and Sean Welsh contributed to this story.