By now it has become apparent that the Romney campaign did indeed receive a post-convention bounce in the polls, somewhere in the range of 2-3%. From what I’ve been able to gather, there are two distinct schools of thought emerging as to how we should interpret the size of that bounce:
The first conclusion is that the bounce is low, especially compared to historical patterns. Nate Silver at 538 explains:
If we take Mr. Romney’s bounce to be two and a half points, it would be the third-smallest for a challenger since 1968, behind John Kerry in 2004 (roughly one point) and Mr. Obama in 2008 (about two). By contrast, the average bounce for the challenger since 1968 is around eleven points.
In a separate analysis, he argued that we should expect to see a bounce of about 4% coming out of the GOP convention, and a smaller bounce should be interpreted as a warning sign to Romney’s prospects this fall.
In contrast, the second conclusion is summarized nicely by Jonathan Bernstein at the Washington Post:
In other words, regardless of the quality of the convention, Clinton was poised for a large bounce because lots and lots of people who “should” have been Democratic voters in 1992 – weak Democrats, a fair number of pure independents, and perhaps even a decent number of solid Democrats – were not with him when the convention started. And what conventions do well is to convert those “should” voters into solid voters.
That’s why no one expected a big bounce for Romney this time, and that’s why it’s unlikely he would have had one even if the Republicans had put on the best show ever. There just wasn’t very much low-hanging fruit; Romney had basically consolidated the Republican vote by early spring.
So basically, Romney didn’t get a large bounce because he had long ago done most of what the bounce does.
Nate Silver also points out that the smallest convention bumps have all occurred in the last three elections (2004, 2008, and now 2012). So, he concludes, perhaps an extremely small convention bounce is now the “new normal” in an era of hyper-partisanship where most people have already made up their minds by the time the conventions roll around.