1976 Carter: ironically despite this being a winning candidacy Carter was up like 30 points on Ford the summer before the election. Ford closed the gap to 2 points on election day. Many pundits have concluded a few more days campaigning and moderate Ford would likely have won this one.
I was in a poli sci course that semester and the course was almost entirely about that election. The data we saw suggested that Ford peaked the weekend before the election, so had it been held on Sunday instead of Tuesday he might have been retained in office. We also astonished the professor by predicting that the election would be one of the closest of the century, which it was.