One of the questions that will be endlessly debated will be whether the convention provided John McCain with a bump up or a bounce (coming back down). You can imagine mechanisms for how a convention could do both. As Patrick pointed out, George W. Bush’s 2004 convention bump gave an enduring lead in the polls.
For example, the Democratic convention likely provided some permanent consolidation of the Democratic base. Hillary Clinton supporting voters probably came home in some part to Barack Obama. These will likely result in a sustained increase in Obama’s floor vote. The convention provided a moment for Hillary-supporting core Democratic base voters to return ot the party’s fold.
Turning to the Republicans, we see a similar mechanism in play that might result in a permanent increase in McCain’s floor. From the new ABC/WaPo poll, we note the cross-tabs of white women
White women have moved from 50-42 percent in Obama’s favor before the conventions to 53-41 percent for McCain now, a 20-point shift in the margin that’s one of the single biggest post-convention changes in voter preferences. The other, also to McCain’s advantage, is in the battleground Midwest, where he’s moved from a 19-point deficit to a 7-point edge.
White women supporting McCain could be attributed to any number of factors. The most obvious is Palin’s star appeal, but also McCain’s focus on service.
But another possibility is that white women are simply coming home to the Republican party. In CNN’s 2004 exit poll, George W. Bush beat John Kerry 55-44 among white women.
In other words, white women voters who should be (or at least easily could be) Republican voters are now back to supporting the Republicans this cycle. Don’t look for this dump to turn into a temporary bounce. This is a real phenomenon with a real mechanism, not some temporary blip caused by a whirlwind of media.