No, it happens,....
Arlen Specter of Pennsylvania
Republican, 1981-April 29, 2009
Democrat, April 30, 2009-2011
Specter was elected as a Republican to the U.S. Senate in 1980, and reelected in 1986, 1992, 1998, and 2004. In April of 2009, he decided to change party affiliation, and became a Democrat on April 30, 2009.
U.S. Senate: Senators Who Changed Parties During Senate Service (Since 1890)