Association with George W. Bush remains a key problem for McCain; likely voters now divide, 49-48 percent, on whether he would lead the country in the same direction as Bush or strike out in a new one. That's not a significant change from last week's 52-45 percent, but it does inch McCain under 50 percent as a Bush 2.0.
Obama, meanwhile, does slightly better than McCain on his own main challenge, experience; likely voters by 54-44 percent say he has the experience it takes to serve effectively as president.
And while likely voters divide closely on whom they trust more to handle an unexpected crisis, 49-45 percent Obama-McCain, that's turned from a 17-point McCain advantage at his high point after his convention.
There are other elements at play.
Sunday and Monday, likely voters overwhelmingly reject Obama's association with 1960s radical William Ayers as a legitimate issue; 52 percent now say McCain's selection of Sarah Palin makes them less confident in his judgment; Obama leads by 2-1 as the more optimistic candidate; and Obama's ratings for his performance across the three debates are substantially better than McCain's, although the final debate did pull up McCain's score.
Then there's the enthusiasm gap, which is far larger than usual.
At the end of the 2004 campaign, Bush's likely voters were more enthusiastic than Kerry's by 9 points, 55 percent vs. 46 percent. In 2000, 44 percent of Bush's supporters were very enthusiastic, 41 percent of Gore's. Enthusiasm for Obama is sharply higher this year.
Much of Obama's advantage lies in greater-than-usual participation by Democrats; they outnumber Republicans by 7 percentage points among likely voters in this poll, as they have consistently this season. In 2004, by contrast, Democrats and Republicans turned out in equal numbers.
Obama's lead depends in large part on a larger-than-usual differential between Democrats and Republicans.
Not only are there more Democrats, Obama's doing a bit better in his own party; 91 percent of Democrats support him, compared with McCain's support among Republicans, 84 percent. Republicans are more often reliable party voters, but 12 percent of them now favor Obama, better than John Kerry or Al Gore's share of Republicans (6 and 8 percent, respectively), and about matching Bill Clinton's in 1996 (13 percent).
Swing-voting independents, meanwhile, divide very closely, 48-47 percent, as do married women, another potential swing group.