WASHINGTON (AP) — Sen. Barbara Mikulski, the longest serving woman in the history of Congress, is announcing her retirement, Democratic officials say.

The 78-year-old Maryland Democrat, now in her fifth term, is set to make a statement at a news conference in Fells Point in Baltimore later Monday. Her retirement could set off a race among potential candidates to succeed her, including Rep. Chris Van Hollen and former Gov. Martin O'Malley, who is considering a bid for president.

Mikulski became the longest-serving woman in the history of Congress in 2012. She was first elected to the House of Representatives in 1976 and has served in the Senate since 1987. Mikulski is up for re-election next year, but has declined in the past to say whether she would run for what would be a sixth term. The deadline for filing is in January 2016.

The officials spoke on condition of anonymity because they were not authorized to publicly discuss her plans.

In her state, she has been fiercely protective of the environment, especially Chesapeake Bay issues.

In a 2014 interview, Mikulski said her approach as chair of the appropriations panel was "to focus with civility and courtesy. Old school values. Don't do surprises or stunts and negotiate directly and not through the press."

Senate Republican Leader Mitch McConnell described Mikulski at the time as forceful and results-oriented. "I think she's terrific," he said.

Mikulski had been seen as more engaging and approachable than her predecessors as appropriations chairman, the late Sens. Robert C. Byrd, D-W.Va., and Daniel Inouye, D-Hawaii. She had spent decades honing relationships with members of both parties, learning their needs and end goals.

"She knows that if you know somebody and what they want, you can help them be successful. And when you help people be successful, Republicans or Democrats, that's how you move bills," said Democratic Sen. Patty Murray of Washington, a Mikulski protege.

In a 2014 interview, Mikulski said her approach as chair of the appropriations panel was "to focus with civility and courtesy. Old school values. Don't do surprises or stunts and negotiate directly and not through the press."

Mikulski has also been an active advocate of equal pay for women. The Maryland senator sponsored legislation last year aimed at tightening a 1963 law that made it illegal to pay women less than men for comparable jobs because of their gender. But Senate Republicans blocked the bill in an April 2014 floor showdown.

"When I hear all these phony reasons, some are mean and some are meaningless, I do get emotional," Mikulski said of arguments against the legislation. "I get angry. I get outraged. I get volcanic."

Mikulski was the latest Democrat to play off former CIA Director Michael Hayden's recent comment that Senate Intelligence Committee Chairwoman Dianne Feinstein, D-Calif., was motivated by "emotional feeling" when she sought an investigation of the spy agency's harsh treatment of terrorism suspects.

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Associated Press writer Laurie Kellman contributed to this story.