Updated at 12:15 p.m. ET.
Sen. Barbara Mikulski, D-Md., who has served in the Senate and in Congress longer than any other woman, says she will not seek a sixth term in 2016.
Mikulski, 78, announced her decision Monday in Baltimore.
" 'Do I spend my time raising money, or do I spend my time raising hell?' " she said she asked herself, according to The Associated Press.
Mikulski was first elected to the House of Representatives in 1976 and has served in the Senate since 1987. She was the first woman to chair the Senate Appropriations Committee. Known as the "Dean" of women in the Senate, Mikulski had a reputation for taking female senators under her wing.
The Baltimore native was a social worker before she was a politician.
The Washington Post, which broke the news of Mikulski's retirement, notes that she is "a forceful presence on many pieces of legislation, passionately liberal on certain issues but also committed to working closely with Republicans."
She's considered to be one of the more liberal members of Congress. She has been a fierce champion of environmental issues and equal pay for women.
In 2012, when NASA discovered an exploding star, they named it "Supernova Mikulski" in her honor.
Her retirement will spark a heated Democratic primary. Potential candidates to replace her include U.S. Reps. Chris Van Hollen and Donna Edwards, and former Gov. Martin O'Malley, who is said to be considering a White House bid in 2016.