You may remember the late Missouri Democrat Senator Thomas Eagleton.
In 1972 he was George McGovern's nominee for Vice President, but withdrew after his psychiatric history was revealed (depression).
So the Democratic National Committee in special session confirmed McGovern's next choice, Kennedy in-law Sargent Shriver.
I've long thought that if Hillary is shown to be losing big-time, she'd get sick, withdraw and be replaced by... who knows, Biden? Kaine? Sanders? Pocahontas?
Of course when Eagleton dropped out it seriously damaged McGovern, so the history of such things is not too promising for the party forced into it.
1972 election results by county:
“In 1972 he was George McGovern’s nominee for Vice President, but withdrew after his psychiatric history was revealed (depression).”
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As I recall, the big issue was that Eagleton had undergone ECT (i.e. “electroshock therapy”).
Interesting, but how do replace the presidential candidate when their name will not even be on the ballot? Don’t forget voting has already started. What if one of the third party’s tried to do such a thing?