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Sen. Barbara Mikulski (D-Md.) is announcing her retirement on Monday, according to The Washington Post and CNN.
Mikulski has served in the U.S. Senate since 1987, and before that, was elected to the House of Representatives in 1976. She has been in Congress longer than any other woman, and was the first woman to head the Senate Appropriations Committee.
Mikulski will make an announcement about her plans at 11 a.m. on Monday.