WASHINGTON – Sen. Barbara Mikulski, the longest serving woman in Congress, says she won’t seek re-election to a sixth term.

The 78-year-old Maryland Democrat made the announcement Monday in Baltimore.

Mikulski known as a tough, no-nonsense lawmaker who rose to the leadership of the powerful Appropriations Committee. She tells reporters that she’ll still have 2 years in her current term and says she hasn’t written “the last chapter” in her career.

Her retirement is certain to set off a race among potential Senate candidates. These include Reps. Chris Van Hollen and Donna Edwards. Others include former Govs. Martin O’Malley, who is considering a bid for the Democratic presidential nomination, and Republican Bob Ehrlich.

Mikulski became the longest-serving woman in the history of Congress in 2012.