Sen. Ben Cardin’s (D-Md.) retirement announcement on Monday is opening the door to what is expected to be a fierce battle on the left to fill a rare open Senate seat in the deep blue state. 

Cardin, 79, revealed on Monday that he would not seek a fourth term in the upper chamber, saying in a video to supporters that “it’s time” for him to call it quits after what will be a 58-year legislative career. 

If the past is any indication, whoever wins the seat could have decades in office. Just five people have represented Maryland in the upper chamber over the past 35 years. 

Democratic sources on Monday indicated that three possible candidates lead the pack of contenders to replace Cardin: Reps. David Trone (D-Md.), Jamie Raskin (D-Md.) and Prince George’s County Executive Angela Alsobrooks (D).

“Those are the big ones I’ve heard,” one Democratic source told The Hill, adding that Alsobrooks has “more than anyone else” been positioning herself to run to replace the longtime Senate Democrat.

“I’ve heard Trone and Alsobrooks’ names floated,” a second Democratic source told The Hill.

Alsobrooks, considered a rising Democratic star in the state, declined to run for governor in 2022 and instead won a second term to lead the second-largest county in the state. At the time, she told The Washington Post that there “will be opportunities to serve on a higher level” in the future.

According to Politico, Alsobrooks has already hired David Chase to manage her run. Chase served as campaign manager for former Rep. Tim Ryan’s (D-Ohio) 2022 bid for the Senate.

A person close to Trone told The Hill the congressman’s team has “said in the past that we’re seriously considering running, we have a record to run on,” adding “Congressman Trone would be happy to serve Maryland, you know, in the U.S. Senate.”

“Nothing’s changed from our perspective a couple weeks ago now that Ben Cardin, now that the senator is retiring,” the person said. “We’re still taking a really close look, and we don’t have any official news just yet, but stay tuned for the days and weeks to come.”

“His No. 1 priority has always been serving Marylanders where he can do the most good,” the person added.

Trone is expected to be formidable in a statewide campaign due in part to his wealth. The founder of Total Wine & More, Trone has dumped millions of dollars into his past congressional campaigns. According to Open Secrets, Trone’s net worth is between $12 million and $33 million.

Trone’s team has already hired Don Morocco as campaign manager. The person close to the congressman said Morocco was brought on “to suss out really what the climate was gonna be.”

But if Raskin and Trone have any interest in a Senate run at all, they didn’t show their cards on Monday. Shortly after the Cardin news went public, both House members released statements lauding the Maryland Democrat’s tenure without any reference to the heated race that will likely take place in Old Line State.

Raskin said in February that he would be open to a run, but he has been battling large B-cell lymphoma since late last year. He announced late last week that his cancer is in remission. 

“Let the races begin,” Sen. Chris Van Hollen (D-Md.) told The Hill when asked how bruising he expects this primary to be. “We’ll see.”

Outside of the trio of Maryland Democrats, others could also look at possible bids for the seat.

Former Rep. John Delaney (D-Md.), who ran for the Democratic presidential nomination in 2020, declined to rule out a possible Senate campaign on Monday. 

“I do think now is the time to only celebrate Ben, his truly magnificent career and his wonderful partner in all of this Myrna — and not the time to talk politics,” Delaney told The Hill when asked about his interest in a bid.

An open Senate seat is a once-in-a-blue-moon occurrence for Maryland, which has not seen an open seat in the upper chamber since former Sen. Barbara Mikulski (D) retired in 2017, ending a 30-year career. Van Hollen succeeded her.

“Democrats have won every statewide federal election in Maryland for the past 40 years, and 2024 will be no different,” David Bergstein, a spokesman for the Democratic Senatorial Campaign Committee, said in a statement. 

On the GOP side, the list of viable contenders is really boiled down to one name: former Gov. Larry Hogan (R).

Hogan, the former two-term governor, was heavily recruited by Senate Minority Leader Mitch McConnell (R-Ky.) to take on Sen. Chris Van Hollen (D-Md.) in 2022, but he passed on a bid, having indicated that he was not cut out to be 1 of 100 and a legislator.  

The GOP moderate also sidestepped a 2024 presidential bid for the Republican nomination. Hogan tweeted out his thanks to Cardin following the announcement and noted that while they “did not always agree, we often worked together as Team Maryland to do what is best for the people of our state.”

When asked about a possible run, a Hogan spokesperson passed along a link to a March story making clear that the former governor is still not interested in running for the upper chamber.

Cardin’s retirement announcement makes him the third Senate Democrat who plans to call it quits at the end of 2024, joining Sens. Debbie Stabenow (D-Mich.) and Dianne Feinstein (D-Calif.).

Stabenow, 73, announced her retirement plans in January, dealing a blow to Democrats who are trying to hold — and grow — their majority in 2024. Republicans are eyeing the seat as a potential pickup opportunity.

Feinstein, 89, revealed in February that she would retire at the end of this term from her safe blue seat. That announcement, however, was made before she came down with a case of shingles, which had her hospitalized in March and has kept her out of Washington since February.

She is now facing calls to resign from some House Democrats, as frustrations linger about the delay in Senate business because of her absence. The Senate Judiciary Committee, which Feinstein sits on, has been unable to advance partisan nominees as she remains sidelined out of Washington. 

Senate Majority Leader Chuck Schumer (D-N.Y.) had said that he wanted Cardin to replace her on the panel temporarily, but the idea was swiftly shot down by Republicans. 

Democrats are defending 23 seats this cycle, including those held by Independent Sens. Kyrsten Sinema (Ariz.), Angus King (Maine) and Bernie Sanders (Vt.). By contrast, Republicans are defending 11 on the ballot next year.

–Updated at 5:35 a.m.