By Sean Trende - September 20, 2012

And of course, when you have a bounce such as this one, which is driven by increased Democratic enthusiasm, you expect to see down-ticket races for House and Senate move toward the Democrats, as more Democrats push through the likely-voter screens. Which is again exactly what we are seeing -- it isn’t accidental that we’ve seen a flood of polling from Democratic House candidates in the past few days, while their Republican counterparts have been relatively silent.

3) History suggests the race will tighten further. Yesterday I mentioned an article by Nate Cohn of The New Republic in which he observed that that “[i]f Romney can’t take a lead over the next week or so, he will be forced to do something never successfully attempted: mount an unprecedented comeback against an incumbent president.” Cohn is absolutely correct: No challenger who trailed at this point in September has ever won.

But if we look at the data a different way, we realize that Obama has to pull off some unprecedented feats of his own if he hopes to win. The September time frame is a bit tricky for comparison purposes, because the incumbent party convention occurred in mid-August until 2004. So a challenger who trailed his opponent in September was doing so after the convention bounces had pretty much settled.

So let’s instead use data that Nate Silver has helpfully compiled identifying where candidates stood a given number of weeks before and after a convention, regardless of when that convention occurred. Table 1 shows the incumbent party’s lead in the polls two weeks after its convention -- roughly where we are today -- and the ultimate result. Years with incumbents are in boldface:

As you can see, no incumbent party has ever held on to the White House while leading by fewer than four points two weeks after its convention; no incumbent president has ever won re-election while leading by fewer than five points (more on the 2004 comparisons later). In other words, winning under these circumstances would be unprecedented (note also that Ronald Reagan was actually tied with Jimmy Carter in a simple poll average at this point in 1980).

Perhaps an even better way to look at this is Table 2:

This lists the races where incumbent presidents sought re-election since 1968. It then shows how those races broke between two weeks after the incumbent president’s convention and Election Day. On average, they moved 3.7 points toward the challenger (positive numbers indicate movement in that direction; negative numbers show movement toward the incumbent). If you eliminate 1976, as Cohn suggests (since Jerry Ford was a pseudo-incumbent), the average movement is six points toward the challenger.

Indeed with the exception of 1992 -- a difficult race from which to draw conclusions given Ross Perot’s on-again/off-again participation in the race -- every contest with an incumbent has broken at least three points toward the challenging party from this point in the race through Election Day.

And given the frequent comparisons to 2004, it’s worth bearing in mind where that race stood at this point. George Bush led by 6.8 points as opposed to Barack Obama’s current 2.9 percent. His bounce peaked at 50.4 percent, as opposed to Obama’s 49 percent. If Obama continues to run behind Bush on either metric by similar margins through Election Day, he loses.