I disagree. In open elections, the most important internal is the favorability rating, which is very similar to but not quite the same (and not quite as predictive) as job approval rating of a sitting President. In 2000, the DUI hit hard at Bush's favorability rating just before the election, turning what was then a 3-4 point lead for Bush into a dead even race in the space of a few days.
When, as here, an incumbent is running for re-election, the undecideds who give the President a favorable job approval rating (and who actually vote) will usually end up voting for him. And vice versa. I'm not making this up -- Gallup has been tracking this type of data for six decades, and they say that the job approval rating is the single best predictor of a President's support on election day.