That's the formula one polisci guy uses to predict the number of seats Democrats are likely to have in the House after the election is over. The argument is that much of the results of midterm elections are structural, that is, based on circumstances apart from any attitude one has about the president. The model predicts that given the nature of the last election and the current economic condition, Democrats should expect to lose 45 seats. Any less means they did well, any more means they did poorly.
Math is fun.
