Fair links the decline of Democrats' fortunes to U.S. economic growth that has been less than 3.2% at an annual rate for all but one quarter since the start of 2009.

'Noticeable Drop'

In January, Fair's model projected a 51.6% share of the vote for Democrats as the economy showed signs of improvement.

"It looked like in January we were maybe going to have some strong growth," he said.

Sluggish growth "knocked off a few percentage points," Fair said. "That's a noticeable drop."

The world's largest economy expanded at an annual pace of 1.6% in the second quarter, the Commerce Department reported on Aug. 27. That was down from an initial estimate of 2.4%, reflecting smaller gains in inventories and a wider trade deficit.

"The economy obviously has an effect on votes," Fair said.

Fair measures that effect by plugging the U.S. inflation rate and the number of quarters preceding the election that economic growth was greater than 3.2% at an annual rate.

One Good Quarter

For the current congressional election, Fair counts one quarter of good growth. To track inflation, Fair uses a measurement that adjusts gross domestic product for inflation, which has averaged .77% since the beginning of 2009. Under Fair's equation, low inflation benefits the incumbent party.