The nine swing states that will decide the election

But political scientists have found that past elections have been more influenced by the changes in the economy in the year or two before the election. And a range of economic data provided by Moody’s Analytics shows that all nine states are rebounding and that most now have unemployment rates below the national average. If voters in those states begin to feel the improvement by the fall and the economy does not take a turn for the worse, it could aid the president’s efforts to hold on to enough of them to win.

But the length and depth of the recession make it more difficult to model behavior, and the slow recovery could complicate things. Xu Cheng, a senior economist at Moody’s Analytics, which uses state economic and political data to predict election results, said his team had altered its model this year to account for “the grumpy voter effect.”

“The so-called grumpy voter effect is that despite economic improvement in a state, if the economic situation in a state is really too bad, the voters will discount the improvement,” Mr. Cheng said. The effect could help tilt foreclosure-racked Florida to the Republicans, he added.