For '68, I think LBJ's reelection (obviously by a much smaller margin than '64) was assumed until the Tet Offensive.
For '80, I think Carter's reelection was assumed until the hostage crisis. Remember how rare it is for a party to lose the WH after just one term.
For '96, I think a close race was assumed. I know there were some close polls at this time, but did Dole ever really lead Clinton?
For '00, uh....no. Gore didn't close the gap with Bush until the general election campaign started. The Lewinsky scandal hurt congressional Republicans (which is why they were trailing in the generic ballot), but it was helping them in their campaign to retake the WH.
Actually Carter approval was 29% before the Hostage crises and it skyrocketed to 63% after the hostages and stayed high until March April 1980 when Carter had already taken down Kennedy