Why tonight matters
Nate Silver reviews post-debate polling from past elections:
There is no evidence, incidentally, that the second presidential debate is any less important than the first one. On average, it has moved the polls by 2.3 percentage points in one direction or another — almost exactly the same as after the first debate, which moved them by 2.4 percentage points on average.
The third debate, however, has often had a more muted impact. The only significant change in the polls following the third debate was in 1992, when Mr. Bush narrowed his deficit with Mr. Clinton, and that may well have reflected a case of reversion to the mean.