Familiarity Breeds ... ?

Well, it depends who you are.

For each day that he spends campaigning in a state in the 30 days in the run-up to the election, Obama can expect to gain about 3.5 points in his margin over Clinton.

And for every day that Clinton spends campaigning in that state, Obama can expect to lose about 2.4 points.

The relative magnitude of the these two numbers is important: Obama is helped more by his own campaigning than he is harmed by Clinton's campaigning. So when both candidates campaign in a state, Obama can expect to gain overall. That dovetails with the finding that Obama tends to move up sharply in the polls in the several weeks leading up to the election.