Post convention bounce and "momentum" are related, but by no means the same thing. SurveyUSA's specialized sampling - which is not a standard preference poll - finds that "momentum" has shifted to Bush:
The number of Americans who think George W. Bush will be re-elected in November has suddenly jumped 10 to 20 points in dozens of cities around the country, according to SurveyUSA tracking polls conducted before, during and after the Democratic and Republican National Conventions.
SurveyUSA has been asking respondents not who they will vote for, but rather: who they think will win the presidential election in November. This question is more sensitive to changes in sentiment, and is designed to capture "momentum" swings more precisely than preference questions asked of likely voters. Tracking polls released today, 9/3/04, the day after the Republican National Convention ended, show sizeable swings in the public consciousness. ....
Results of these surveys are not intended to forecast the outcome of the presidential election in November nor predict how respondents would vote in an election "today"; rather, results here are intended solely to measure shifts in momentum, which can reveal much about the public's collective psyche. Momentum can and does change in a campaign, sometimes dramatically. Events may well happen during this campaign to shift momentum back to the Democrats just as suddenly as it has here shifted to the Republicans. Results presented here should not be confused with SurveyUSA Election Polls, which are conducted over a longer field period, and which tightly screen for registered and likely voters, and which do in fact ask respondents how they would vote in an election "today."