Have Humphrey win in '68 and lose to Reagan (who was much more extreme earlier in his political career) in '72. Then, somehow, have the mid 70s fare better economically and have him get re elected in 76. A Democrat wins in 1980 over a moderate Republican after a second Reagan term goes similarly to Carter's OTL, but said Democrat isn't able to bring the economy back like in OTL and loses big in 1984.