The only reasonable way Goldwater, McGovern or Mondale could have won would have been for the incumbent to unexpectedly die in office.
Now, for 1984, if Hinkley had assassinated Regan, it is possible a moderate like John Glenn could have defeated President Bush. Keep in mind one of Regan's assets was charisma - something totally lacking in Bush.
Inspired by Douglas' thread:
With a POD no earlier than March 31, 1981 have the Democratic Party sweep the 1984 elections. The Democratic candidate for President must win in the Electoral College.