Barring a career-destroying scandal, it’s hard to come up with a scenario (aside from very popular third party candidate) such that either major party candidate drops much more than a point or 2 below 40% in the general election. In those landslide Republican wins, Carter got 41%, Mondale got 40.6%, and Dukakis got 45.6%.
The Mondale loss actually was a landslide win for the Republicans. He did worse in the electoral college than George McGovern in the 1972 election (13 for Mondale vs. 17 for McGovern).
And that point or two below 40%? McGovern got 37.3% of the popular vote.