WASHINGTON (AP) — The longest serving woman in the history of the U.S. Congress said Monday she won't seek re-election to a sixth term.

Sen. Barbara Mikulski, a Maryland Democrat, made the announcement in Baltimore.

The 78-year-old's retirement is certain to set off a race among potential candidates to succeed her, including former Democratic Gov. Martin O'Malley, who is considering a bid for president, Republican Bob Ehrlich and Democratic Reps. Chris Van Hollen and Donna Edwards.

A Democrat is favored to win the seat in the liberal-leaning state.

Mikulski, now in her fifth term, 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 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.

Mikulski was known as a tough, no-nonsense lawmaker who rose to the leadership of the powerful Appropriations Committee. She spent decades honing relationships with members of both parties, learning their needs and end goals.

She has also been an active advocate of equal pay for women. The 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.

Mikulski told reporters that she'll still have 2 years in her current term and says she hasn't written "the last chapter" in her career.