Examples that come to mind:
John W. Davis (1924 Dem Nominee) voted GOP for President in 1936, 1940, and 1944
Al Smith (1928 Dem Nominee) voted GOP for President in 1936 and 1940
George McGovern (1972 Dem Nominee) voted for Gerald Ford in 1976 (this one still makes no sense to me)
George HW Bush voted for Hillary Clinton in 2016
Are there any other examples? I know that Grover Cleveland supported 3rd party candidate John Palmer in 1896, but I don't know who he supported in 1900.
Source: nytimes.com/1952/10/23/archives/eisenhower-gets-support-of-davis-24-democratic-candidate-for.html
Also, while not technically their party's nominees, I looked into how Strom Thurmond and George Wallace voted after '48 and '68.
Thurmond after 1948 voted for Ike in 1952, South Carolina's unpledged slate in 1956, and the Republican nominee from 1964 onward. The only one I couldn't find for him was 1960. I suspect privately he might have voted for Kennedy, though I am quite unsure.
George Wallace after 1968 voted for Nixon in 1972, Carter in 1976, Bush in 1992, and Dole in 1996. I couldn't find how he voted in the 1980s, but I would be surprised if he didn't vote for Carter in 1980. Very unsure about 1984 and 1988 for him.
Sources:
Wallace for Nixon 1972: johngschmitz.com/_files/koplinski.pdf
Thurmond for Ford 1976: livingroomcandidate.org/commercials/1976/strom-thurmond
didn’t Wallace imply toward the end of his life he voted republican?
