2008: Al Gore/Joe Biden vs. John McCain/Joe Lieberman (no stock market crash)

[President Johnson]

Stock market crash doesn't happen before the election and Al Gore runs again for president. He defeats Hillary for the nomination while Obama sits the election, eying a gubernatorial bid in 2010. Al Gore picks Joe Biden as running mate while John McCain resists pressure from the right and goes with Joe Lieberman, who was once Gore's running mate. What happens?

[President Johnson]

Tilt Gore. Even outside the crash Bush's unpopularity still would've been a problem.

Mostly because of Iraq, it seems. However, even a popular two term president doesn't automatically translate into a victory of his party's nominee. See Eisenhower, Clinton and Obama, who were all solid above water on the eve of the election to pick their successors. 1988 is sort of an exception, though I think even that one was winnable if Democrats chose a better candidate than Dukakis.