1960 - Eisenhower
1988 - Reagan
2000 - Clinton
1968 - Johnson
who could have run, constitutionally, since he had served less than 50% of JFK's term after the assassination, but was such damaged goods from the Vietnam War that he bowed out.
For completeness's sake, the other races were elections where the incumbent was either re-elected (Eisenhower - 1956, Johnson - 1964, Nixon - 1972, Reagan - 1984, Clinton - 1996, Bush II - 2004) or defeated for re-election (Ford - 1976, Carter - 1980, Bush I - 1992).
For those four, the significant thing is that each one had a vice-president who was trying to get elected to succeed him. Namely:
Nixon to succeed Eisenhower (lost)
Humphrey to succeed Johnson (lost)
Bush I to succeed Reagan (won)
Gore to succeed Clinton (lost)
Each of those four outgoing presidents knew that after the final mid-term election of his second term, he himself would never again have to face the judgment of the voters. But each also knew that his closest political teammate, his vice-president, would. And that, furthermore, the fate of that vice-president would be a form of referendum on the legacy of the outgoing president. This had a certain restraining effect on the president's actions after that final mid-term.
But in 2008, for the first time in decades, the outgoing president will have no heir apparant. Cheney can't run, and isn't considering it. He's too old, he's got a weak ticker, and even a lot of Republicans consider him just a bit too creepy. There will be other Republicans vying for the nomination, but Bush has no close ties to any of them. It will be wide open.
So what restraining effect will there be on the Bush White House after tomorrow, however the vote goes? What will be the administration's attitude?
If the Republicans hold onto both houses of Congress, the temptation will be: "Woo hoo! We won! We can do anything we want now! BWA-ha-ha-ha-ha!"
If they lose either or both houses, the temptation will be: "Fuck it. We lost. We can do anything we want now. We have nothing left to lose."
I don't know which is more dangerous. Whatever happens tomorrow, we're in for a few years of interesting times.