In their look at the 2012 election, our John Sides and UCLA's Lynn Vavreck found that ads made a difference in the race when the balance was lopsided, as it is now. They also found that the presence of staff on the ground made a slight difference in the margin for a candidate in that region. (Without his field operation, they estimate, Obama probably would have lost Florida.) It's very early; Sides and Vavreck also found that ads right before the election made the biggest difference.