The Incumbency and National Conditions Model predicted that John McCain would receive 44.3% of the two-party presidential vote; he ended up with 46.6% (at this writing, November 13, 2008), yielding a prediction error of 2.4 points. This error is slightly smaller than the mean absolute value of the out-of-sample forecasts from 1952 to 2004 (2.68), so the 2008 election result is not anomalous in the context of this model. The addition to the model of the 2008 outcome does not generate many important changes to the coefficients, except that the significance of incumbency interaction went from 0.08 to 0.02 and the standard error and adjusted R² of the model both indicate a slight improvement in fit.

The underlying assumption of this model was that the incumbent party would not be held fully responsible for the low levels of presidential approval and equally low levels of satisfaction with personal finances because the president himself was not on the ticket. Given that the outcome of the election does not fall far from the prediction generated by the model, and that the incumbency interaction term was strengthened slightly by the addition of the 2008 outcome, the Incumbency and National Conditions Model is on sound footing.