It's unlikely that economic growth in the third quarter will be strong enough to give Democrats a bounce, and inflation won't change enough to affect his formula's result. Voters may respond to worse economic news.
"What happens between now and the election on the economy obviously does have some effect on the vote, it's just that it's too fine of changes for me to pick up," he said.
Political analyst Charlie Cook yesterday predicted that Republicans would gain at least 40 seats and take control of the House.
Fair doesn't try to figure out what his vote breakdown would mean seat-by-seat in the House. He said the vagaries of individual House districts make that difficult. Still, the overall percentage can be useful.
"The percent of a two-party vote of the House is not a bad measure of what I'm after," he said. "It's an indication of what people think of the two parties."