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.
I doubt that. The GOP would have tried to beat RFK at his own game and chosen a centrist like Rockefeller.
In any case, Rockefeller probably would have lost, causing an internal party conflict between liberal and conservative Republicans (aka Rockefeller Republicans vs Goldwater Republicans). Reagan would then have won the nomination for 1976, coming up against someone like Henry M. Jackson or Morris Udall, possibly losing. 1980 sees liberal Republican John Anderson winning the Presidency. In other words...
1968: RFK/Sanford bt Nixon/Agnew
1972: RFK/Sanford bt Rockefeller/Ford
1976: Jackson/Vanik bt Reagan/Schweiker
1980: Anderson/Dole bt Jackson/Vanik
1984: Anderson/Dole bt Hart/Mondale
1988: Bentsen/Gore bt Dole/Bush
1992: Bentsen/Gore bt Bush/Kemp
1996: Gore/Gephardt bt Quayle/Alexander
2000: McCain/Lieberman bt Gore/Gephardt
2004: McCain/Lieberman bt Edwards/Richardson
Presidents since 1969
1969 - 1977 Robert F. Kennedy
1977 - 1981 Henry M. Jackson
1981 - 1989 John Anderson
1989 - 1997 Lloyd Bentsen
1997 - 2001 Al Gore
2001 - Now John McCain
Something like this? Btw, Lieberman would have been too conservative for the Dems in this scenario (since the political centre would have gravitated more towards the left), thus running alongside McCain in the 2000- and 2004 GOP tickets. As for the USSR, it probably would have collapsed at some point during the Bentsen Presidency as it was already starting to self-destruct in the early 1970s anyway.
