Some elections are about persuasion. This one is about turnout. In past presidential debates, candidates have tried to change people's minds and win over undecideds. In this year's presidential and vice presidential debates, the candidates have tried to increase the enthusiasm of their base.
The results have shown up in the polls, most vividly in those polls that don't weight the results to match the party identification of the last couple of elections.
Thus, George W. Bush zoomed up in some polls right after the Republican National Convention. The first post-convention Gallup poll had him ahead of John Kerry by 52 percent to 45 percent, compared to a 48 percent to 46 percent lead before the convention.
Gallup is volatile because its likely voter screen is very tight, so that not much more than a majority of registered voters are classed as likely voters. The post-convention sample had more Republicans and fewer Democrats than the pre-convention poll. That's evidence that Republicans felt a lot more enthusiastic about their president after the convention than before.
Similarly, Gallup showed Kerry spiking to a 49 percent to 49 percent tie after the first debate, as compared to a 52 percent to 44 percent Bush lead in Gallup's last pre-debate poll. That's evidence that the balance of enthusiasm tilted toward the Democrats after the debate.
Pollster John Zogby, who weights his results by party identification, shows much less movement. Starting with a poll taken during the convention, Zogby has shown Bush at 46 percent and Kerry between 42 and 45 percent.
Who is right? The answer, I suspect, is somewhere in between. Zogby has got it right in one sense: very few Republicans are abandoning Bush, and not many more Democrats are abandoning Kerry.
But Gallup also has a point. Republican and Democratic turnout is not the same year after year. The 2000 VNS exit poll sample was 39 percent Democratic, 35 percent Republican. But in 2002, the turnout was apparently more heavily weighted to the Republicans, though we're not sure because the exit poll was flawed and not conducted in every state.