These are very bad examples to use when trying to make a historical point. The GOP was doomed in 1976 because it was the first election after Watergate -- and Ford couldn't even run as a credible incumbent because he hadn't even been elected as Vice President, let alone President.
If anything, 1992 and 1996 confirm the enduring popularity of conservatism in this country. Bill Clinton would have spent the last 13 years living in a trailer in Arkansas if it hadn't been for Ross Perot's third-party run in 1992 -- and Clinton couldn't even get 50% of the popular vote at the height of his popularity in 1996.
And why did clinton or perot have a chance to knock bush out of it in 1992?
Because bush abandoned his base. Raising taxes, ADA, 89 gun ban, you name it.
He made the power base of the party start to doubt why they were out their volunteering, giving money, and more.
When that happens, you're SOL.