In 1944, there was talk that FDR might choose a liberal Republican like Wendell Willkie as his running mate. In the same year, "Some Republicans wanted to ask Democratic Senator Harry Byrd of Virginia to be the Republican running mate in order to pursue the Southern vote, but this possibility was not seriously pursued." https://en.wikipedia.org/wiki/Republican_Party_vice_presidential_candidate_selection,_1944
In 1956, there was talk that Ike might dump Nixon in favor of a conservative Democrat like Frank Lausche.
In 1972, there was talk that John Connally, then still nominally a Democrat, might replace Spiro Agnew as Nixon's running mate.
In 2008, there was talk of a McCain-Lieberman (much more talk than of an Obama-Hagel) ticket.
In 2016 there was talk that Trump might name a Democrat (Jim Webb was the one most often mentioned) as his running mate.
Now there are various reasons none of these things came about, and the fact that something that is frequently talked about never happens doesn't mean it never could. But as with brokered conventions, one begins to suspect after a while that there must after all be a reason it never seems to happen....
In 1956, there was talk that Ike might dump Nixon in favor of a conservative Democrat like Frank Lausche.
In 1972, there was talk that John Connally, then still nominally a Democrat, might replace Spiro Agnew as Nixon's running mate.
In 2008, there was talk of a McCain-Lieberman (much more talk than of an Obama-Hagel) ticket.
In 2016 there was talk that Trump might name a Democrat (Jim Webb was the one most often mentioned) as his running mate.
Now there are various reasons none of these things came about, and the fact that something that is frequently talked about never happens doesn't mean it never could. But as with brokered conventions, one begins to suspect after a while that there must after all be a reason it never seems to happen....