Cardin was elected to succeed Paul Sarbanes in the 2006 U.S. Senate election, defeating Republican Michael Steele, the Lieutenant Governor of Maryland, by a margin of 54% to 44%. He was reelected in the U.S. Senate election, 2012 taking 56% of the vote. He became a senior U.S. Senator on January 3, 2017 upon Barbara Mikulski's retirement. Cardin is running for reelection to a third term in the U.S. Senate election, 2018.