George H.W. Bush lost his presidency in 1992 with an unemployment rate scarcely different from Reagan’s in 1984. Why? Because the rate had accelerated from the 5.3% when Bush was first elected. Ronald Reagan’s 1980 debate line resonated anew in 1992, to the detriment of his chosen GOP successor: “Are you better off today than you were four years ago?”

And look at the flip side of the employment coin. If unemployment were the alpha and the omega of presidential politics, then the Democrats would have won landslides in 1968 and 2000. Instead, Hubert Humphrey and Al Gore lost (the latter, one admits, on a technical knockout). HHH had as strong an employment picture as a candidate could hope to see; the nation was at almost full employment (3.4% unemployment). But the Vietnam War dominated the election, and produced the Nixon presidency. In 2000, ethics and personality arguably counted for as much as the Democrats’ low 3.9% unemployment rate, getting George W. Bush within striking range…

If you insist on looking at just one number for an incumbent president seeking reelection, it is probably best to choose presidential popularity — the answer to the question, “Do you approve or disapprove of the job President X is doing?” This is a marvelous summary statistic, because it forces voters to take all the issues of the day into account, projecting their own issue priorities onto the president. When reliable polls show the proportion approving of presidential job performance is 50% or above, the incumbent is highly likely to win. The further the number falls below 50%, the less likely the incumbent is to get his second term.