In '68 the Dems overperformed, Humphrey shouldn't have made it that close, with the war and LBJ.
In '72 they underperforned but it was mainly in nominating McGovern and with Eagleton.
In '76 Carter should've won 400+ EVs.
In '88 we should've won much more, if not the election.
In '08, the GOP overperformed, Obama should have won around 400 EVs with the economy as it was.
The rest were pretty much what could be expected.
The GOP over performed and prevented Obama from getting 400+ EV's because their vote was pretty resilient. Losing only three million voters while had it been in the past Obama would had won 10%+ and 400+ EV's easily.
Obama was a very poor fit for TX and the Deep South compared to past Democratic nominees. This kept it from being an electoral blowout in 2008.