The second bit of evidence comes from the Gallup/USAToday poll taken July 25-27. This poll showed that when you narrowed the base of respondents down from registered voters to likely voters, John McCain was ahead 49 percent to 45 percent. That's a vivid contrast from the contemporaneous Gallup and Rasmussen tracking polls, and it was the first national poll since May showing McCain ahead.

The Gallup/USAT poll employs an unusual technique to decide who is a likely voter, and accordingly its results tend to vary more widely than other polls; political insiders tend to take its numbers seriously less as an indication of where the race is than as an indication of which candidate is benefiting, at least for a moment, from the balance of enthusiasm.

For most of this year, the balance of enthusiasm has been in favor of Democrats and Obama. Turnout in Democratic primaries was about 50 percent higher than in Republican primaries while both parties' nominations were seriously contested; Democrats generally and Obama in particular have raised far more money than Republicans; McCain voters have typically expressed less enthusiasm for their candidate than Obama voters have for theirs.

These poll results suggest that something -- the rantings of the Rev. Wright or Obama's skinbacks on issues like Iraq and terrorist surveillance -- has dampened enthusiasm for him, particularly among the young. The hope that his candidacy would benefit from a historically unprecedented turnout of young voters seems more audacious than it did a short time ago.

Nothing about an election is harder to predict than turnout, and experience shows that the balance of enthusiasm can change abruptly and in unpredicted ways. The poll numbers examined here are of course not the final word, or anything close to it, and this campaign could take many twists and turns before it is over. But you could do worse than expected the unexpected.