Has a sitting US president, or past president, ever endorsed or voted for the candidate for president from the other party?

by lingben

This is with regards to the recent news of G.W.H. Bush voting for Hillary Clinton. Is this unprecedented?

Mddcat04

John Adams (Federalist) supported his son's run in 1824 as a Democratic Republican. Not really the same situation though, as the Federalists had effectively ceased to exist by that time. The election of 1824 featured 4 candidates all of whom were (at least nominally) Democratic-Republicans.