I am going to guess that just about everyone chose No. 10 (2008) and No. 11 (1980) as years where the incumbent party “should” have lost, and that most people also chose No. 1 (1992). I am also going to guess that very few people thought that Nos. 3 (2000), 4 (1988), 7 (1996), 9 (1972), or 12 (1984) were years that the incumbent party should have lost.
That leaves us with Nos. 2 (2004), 5 (1968), 6 (2012) and 8 (1976) as years that probably could have gone either way. As it turns out, these were all relatively close elections, which might have turned out differently but for a few external factors.
We’ve done this without any reference to party ideology. Unless your conclusions were radically different that what I’ve described, you’ve discovered a huge shortcoming with the basic narrative that the Democrats were too liberal to win from 1968 to 1988, and began winning only when they ran to the center. The truth is, from 1968 through 1988, Republicans had some pretty good luck with the playing fields. They won a close election in 1968 that the economic fundamentals suggested could have gone either way, and lost a close election that could have gone either way in 1976, but the rest of the elections would have been significant surprises had they turned out any differently. Put differently, if we can explain these elections well without reference to ideology, why put ideology in the discussion (we’ll talk more about this when discussing question No. 4 and in the conclusion).
From 1992 through 2012, Republicans barely won one election (in 2000) in an environment where they really didn’t have any business being competitive in the first place. They have split the elections that could have gone either way. Otherwise, they’ve had the misfortune of running in some pretty lousy environments.