It's the unemployment rate.Want to know something? When Jimmy Carter beat Gerald Ford in 1976, the unemployment rate was 7.8 percent. When Ronald Reagan beat Jimmy Carter, the unemployment rate was 7.5 percent. When Bill Clinton beat the senior George Bush, the unemployment rate was 7.4 percent.Does recent history suggest that high unemployment means you get booted from the American presidency? Yes, but…The “but” was 1984, when the rate was almost as high as these numbers (it was 7.2 percent), but had come way down from double-digit levels the year before. It was truly “morning in America” in that Reagan re-election year because it had been a nightmare so recently.So here we go. It's July 2010. We've got two and half years to go 'til November 2012. If Barack Obama can get unemployment heading dramatically down, people will forgive him for it still being in the seven-percent level. If not, history shows that they will not. The only question, then, is whether the Republicans can mount a credible candidate to run against him.This is the Republican formula: High unemployment holding and a credible candidate for President. Think of Richard III: “My kingdom for a horse.” If they've got the horse, and Obama's got high unemployment, bet on the Republicans. That's what history teaches.The Obama formula: Find a means of getting the American economy onto a growth arc. Get people buying and business-investing; get the wheels of capitalism turning; get the demand for goods and services rising. If that doesn't happen, all the Republicans need to do is find a horse.That said, finding a horse in a party divided by Tea Party zealots and regular Republicans might be almost as hard as getting the jobless rate down.Anyway, I think I've just defined the competition.