Rather than seek a full term in 1936, Benson ran for governor as a Farmer-Labor candidate and was elected. Oregon's Wayne Morse was a two-term GOP senator when he decided to leave the Republican Party in 1952 and become an Independent. In 1955 he switched to the Democratic Party and was elected two more times before his defeat in 1968. Many thanks to the Senate Historian's Office and the National Governors Association for their help on this answer. Mich. The last GOP senator to leave his party was Oregon's Morse in 1952.
Nobody comes to mind.
There has always been talk that some pro-abortion rights Republicans might feel more comfortable in the Democratic Party. But many of them stay in the GOP because of other issues, such as taxes or government spending.