WASHINGTON — Sen. Barbara A. Mikulski's startling announcement Monday that she will not seek re-election in 2016 after more than four decades in elected office set off a political free-for-all as Maryland's most powerful politicians began to position themselves for the opportunity to run for a rare open seat.
Just hours after the dean of Maryland's congressional delegation shared her decision at an emotional news conference at Henderson's Wharf in Fells Point, several Democrats said they are considering a run for the seat, and many more declined to rule one out — setting up the possibility of a bruising primary that will be among the most closely watched in the nation.
Former Gov. Martin O'Malley, who has been speaking to voters in Iowa and New Hampshire and raising money in preparation for a possible bid for the White House, did not comment Monday on whether the announcement by his onetime boss would cause him to change his plans.
But an aide to Rep. Chris Van Hollen, long considered a rising star in the party, said the congressman is "very likely" to run for the Senate. Reps. Elijah E. Cummings of Baltimore and John Delaney of Montgomery County said they are looking at it, too. Aides to Anthony G. Brown, O'Malley's former lieutenant governor, said he is seriously considering a run.
And Baltimore Mayor Stephanie Rawlings-Blake has been elevating her national profile and, observers said, could pose a formidable challenge.
"It's not like we have to recruit candidates," Sen. Ben Cardin quipped. "It will get a lot of attention."
Several Republicans were also looking at the seat. They included Rep. Andy Harris of Baltimore County — the state's only Republican elected to federal office — and former Secret Service agent Dan Bongino, who secured the GOP nomination for Senate in 2012 but lost to Cardin in the general election. Some Republicans also floated Robert L. Ehrlich Jr. as a possible candidate, though the former governor declined to discuss the idea.
Some Democrats voiced concern about the effect a competitive, expensive primary would have on a party still reeling from Gov. Larry Hogan's upset victory over Brown in November. Republicans, as if to reinforce the point, were quick to suggest a messy contest could give the GOP an opening in a state that has not elected a Republican to the Senate since 1980.
"It's going to be a donnybrook," state Senate President Thomas V. Mike Miller said. "It creates turmoil down the entire chain."
The Prince George's County Democrat said the party is particularly vulnerable after losing the race for governor: "There is no unity. There is no party boss. There is no party discipline.
"It's a free-for-all," he said.
Mikulski, a former Baltimore social worker, has been politically invincible since she was first elected to the House in 1976. The longest-serving woman in Congress, Mikulski won the 2010 primary with 82 percent of the vote. She has crushed GOP opposition in successive general elections.
The senator's stature in the state is so pronounced that many in Maryland scoffed at rumors swirling in Washington of her retirement, even though she had not raised significant money in months.
It didn't matter, many said: Polls have long shown she is the most popular politician in Maryland.
But Mikulski, who turns 80 in 2016, said she felt she could be more effective serving out the next two years without the distraction of a campaign.
"Do I spend my time raising money or raising hell to meet your day-to-day needs?" Mikulski said. "For the next two years, I will be here, working the way I do, 100 percent. Fighting the way I do, being strong in principle but trying to build those bipartisan coalitions. And when this term is done, I will know I have given it my very best shot."
Asked about potential successors, Mikulski demurred.
"Maryland has a lot of talent," she said. "And they'll be telling you about it in the next 10 minutes."
It was another shrewd assessment from someone who has been a fixture of Maryland politics since the early 1970s.
Delaney, a two-term lawmaker from Potomac, said he would explore a run. His candidacy would be significant for two reasons: He is wealthy enough to throw in millions of dollars of his own money at the race, and he has taken an outsider, centrist approach in Congress that would set him apart from other state Democrats.
Hours later, an aide to Van Hollen said the popular seven-term congressman was likely to seek the seat. Van Hollen, the top-ranking Democrat on the House Budget Committee, is widely considered a savvy political operator. He would have no trouble raising money and has built up a national network of support after leading the Democratic Congressional Campaign Committee during the 2008 and 2010 election cycles.
The race will undoubtedly be the most expensive in state history. In the 2012 race for Senate in Massachusetts — a state, like Maryland, long dominated by Democrats — Elizabeth Warren, a Democrat, and Republican incumbent Scott Brown spent more than $70 million, according to the Center for Responsive Politics. Warren won that race.
Though Rawlings-Blake has not raised significant money, she has worked to elevate her national profile, taking on a leadership role in the Democratic National Committee, appearing frequently on the Sunday talk shows and often representing large-city interests at the White House.
The mayor's office issued a statement saying Rawlings-Blake is focused on the city, but aides wouldn't rule out a possible run for Senate.
Brown demonstrated an ability to raise money, and likely has high name recognition after just having run a statewide race. But he lost in a major upset — and it might be difficult for him to make a case of his electability so soon after.
"Today, in an increasingly divided nation and dangerous world, it is more essential than ever that Maryland elect someone who can continue Senator Mikulski's legacy of fighting for all Americans, keeping our nation safe, and getting results for the people of our state," Brown said in a statement.
Other Democratic names floated on Monday included U.S. Labor Secretary Tom Perez, who lives in Montgomery County and has deep political ties to Maryland, and Rep. Donna Edwards of Prince George's County. Rushern L. Baker III, the Prince George's county executive, did not rule out a run.
"Whenever you have these kinds of once-in-lifetime opportunities, you've got to take a look at it — and that's what I plan to do," said Cummings, who has raised his own profile as the top-ranking Democrat on the House Oversight and Government Reform Committee.
"We have a deep bench," Cummings added, "but one of the things that we do have to be concerned about is making sure that we have a candidate who will not only be able to win the primary but will be able to win the general."
The filing deadline to run for Senate from Maryland falls in mid-January. That means O'Malley would have to make a decision on the seat before voting begins in the early presidential primary states. Polls show he has been unable to gain much traction against presumed front-runner Hillary Clinton.
A Senate seat would be an easier climb for O'Malley, 52, and would allow him to keep his name in the mix for future presidential elections.
O'Malley told The Baltimore Sun in 2011 that he wasn't particularly interested in a Senate seat. He suggested he would find it difficult to work in a legislature after serving as an executive.
The former governor, who worked on Mikulski's 1986 campaign, released a statement Monday calling her "my friend and inspiration."
An O'Malley spokeswoman said it was a day to reflect on Mikulski's legacy, "not engage in political speculation."
The primary is scheduled for April 5, 2016.
Hogan surprised the political establishment — and everyone else — with his win last year in a state where registered Democrats far outnumber Republicans. Some speculated the victory could give a boost to potential GOP Senate candidates in 2016.
But even some Republicans acknowledge the political dynamics will be significantly different in a presidential election year. Many Democrats who stayed home in 2014 are likely to turn out for an election to fill a vacancy in the White House.
Still, open Senate seats are rare, and some Republicans — including Harris and Bongino — will likely take a hard look at the seat.
"It's surprising news. We wish her well," an Ehrlich spokesman read from a statement from the former governor. "No further comment."
Michael S. Steele, the former lieutenant governor and chairman of the Republican National Committee who ran against Cardin in 2006, responded jauntily to a tweet encouraging him to enter the 2016 race.
"Paging @MichaelSteele," a supporter wrote to the news of Mikulski's retirement.
"Hello?!" Steele responded.
Whoever runs on the Republican side, party officials indicated that they see opportunity.
"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 National Republican Senatorial Committee, the campaign arm of Senate Republicans.
"While Democrats get ready for a bloody primary," she said, "we will have a top recruit waiting for whoever emerges."
Baltimore Sun reporters Erin Cox, Michael Dresser, Doug Donovan, Yvonne Wenger, Luke Broadwater Meredith Cohn and Kevin Rector, contributed to this article.