Debates normally have a modest impact

First Debate Often Helps Challenger in Polls 

By NATE SILVER, NYT

Conventional wisdom holds that the first presidential debate offers an especially good opportunity for the challenging candidate, who for the first time gets to stand on a literal public stage, and a proverbial level playing field, with the incumbent president.

As much as we like to debunk the conventional wisdom at FiveThirtyEight, this hypothesis has the ring of empirical truth to it. There are no guarantees for Mitt Romney, and if he makes gains in the polls following Wednesday night's debate in Denver, they will probably be fairly modest. But if historical precedent is any guide, he is more likely than not to see his standing improve at least some.

This analysis will be quite simple: I've made a comparison of the polls just before and just after the presidential debate in years dating back to 1976.

The polls in the "before" row consist of a simple average of all polls that ended their interviewing within a week before the first debate. The polls in the "after" row consist of those that began their interviewing in the week that followed it. Polls that overlapped with the debate are excluded.

(More here.)