
The idea here is to give people a reference point for what the public perceives as acceptable economic performance from a president. Obviously, Carter's performance in 1979-80, with high unemployment and high inflation, didn't wow enough people. But notice that in 95-96, Clinton's numbers were just solid, not spectacular -- better than Bush's right now, but not dramatically so, and Clinton got re-elected fairly easily. Bush's daddy was also hurting bad when Clinton dethroned him. But what's really interesting is Reagan in 1984. Notice the giant unemployment rates -- 8.3 and 7.3 in the years prior to and during the election.
But things were perceived to be getting better. That's the trick. People felt that the country was moving in the right direction, or at least felt the odds of that happening were worth not risking hiring someone new and untested to replace Reagan. And things eventually worked out, at least on the short-term economic front. There is a tendency for the public to stick with the evil it knows, thus the incumbency advantage, and to be reasonably smart about judging its own economic circumstances.
Let's go back to LBJ in 1967. His numbers were enough to get him re-elected, had he been able to tough it out in the primaries. He chose to cut and run, and we got Nixon as a result. Clean for Gene...