Given Mr. Obama’s current approval ratings and consensus forecasts on the economy, he rates as about a 60 percent favorite to win the popular vote against Mitt Romney, with somewhat higher chances against any of the other Republicans running for the nomination. By contrast, in the November version of the model, Mr. Obama was an underdog to Mr. Romney, with a 40 percent chance of winning; the president’s approval ratings were about 6 points lower then and economic forecasts were somewhat more pessimistic.