Mr. Obama remains slightly ahead of Mr. Romney in most national polls, and he has had a somewhat clearer advantage in polling conducted at the state level. Mr. Obama would be about 80 percent likely to win an election held today, according to the model.

However, the outlook for the Nov. 6 election is much less certain, with Mr. Obama having winning odds of just over 60 percent. The forecast currently calls for Mr. Obama to win roughly 290 electoral votes, but outcomes ranging everywhere from about 160 to 390 electoral votes are plausible, given the long lead time until the election and the amount of news that could occur between now and then. Both polls and economic indicators are a pretty rough guide five months before an election.

The forecast works by running simulations of the Electoral College, which are designed to consider the uncertainty in the outcome at the national level and in individual states. It recognizes that voters in each state could be affected by universal factors — like a rising or falling economic tide — as well as by circumstances particular to each state. Furthermore, it considers the relationships between the states and the ways they might move in tandem with one another. Demographically similar states like Minnesota and Wisconsin, for instance, are more likely to move in the same direction than dissimilar ones like New Hampshire and New Mexico.