According to historian Richard Norton Smith, some Democrats including Hubert Humphrey, raised questions about a possible brokered Democratic convention in 1976. In such a scenario, the possibility of asking Nelson Rockefeller to switch parties and run as the Democratic nominee was given consideration. What if that plan had succeeded and Rocky, as sitting vice president, would run against President Ford in the November election?
Norton Smith talks about that here (at 3:25 minutes; the link already starts there):
https://www.youtube.com/watch?v=Zd_DNA9ozmI#t=3m25s