In 1972, had Kennedy turned out to be as described in your scenario, corrupt, authoritarian, and ineffective, yes, Nixon would have defeated him. However, by 1972, with Nixon losing twice arleady for the Presidency, I would hope the Republicans would have moved on to a better candidate.
Seems to me that Ronald Reagan would be the obvious choice - the anti-government, anti-counterculture California governor.
Not yet, I don't think the Republican base would be ready for Reagan in 72. I think they might try out Rocky or Dole first.
