Richard Nixon declined to debate in 1968 and 1972 (there was also no TV debate in 1964 between LBJ and Goldwater), probably in no small part due to his infamous performance in the debate in 1960 with JFK, which has come down as popular wisdom as the epitome of not only the power of television (it was the first televised debate) but also how to misunderstand the medium (i.e. Nixon's sweaty upper lip with no make up contrasted with the movie-star-like JFK).

There are completely valid challenges to whether or not that received wisdom was actually correct. I'd love to hear from more of a Nixon scholar or US politics scholar about whether Nixon ever commented or wrote (or if any of his campaign team ever commented or wrote) about the decision not to debate, but in any event every presidential election since then has had the participation of the candidates in at least one televised debate.