McCain, though, has troubles of his own: His profile includes a substantial "negative" vote, which can be harder to turn out at the polls -- 30 percent of his supporters are more anti-Obama than pro-McCain. Obama's support, by contrast, is just 18 percent negative. (John Kerry's support was especially negative -- at its peak, 61 percent anti-Bush rather than pro-Kerry.) And Americans only divide evenly on whether McCain shares their values; by contrast, they say by 56-39 percent that Obama does.
McCain's also hurt by an unusual Democratic edge in partisan allegiance: In this poll 37 percent of Americans identify themselves as Democrats vs. 24 percent Republicans, an effect of George W. Bush's broad unpopularity. (Partisanship is 37-27 percent among registered voters, but tightens to 33-32 percent among likely voters.)
Bush, indeed, is a major millstone for McCain. Given the unpopular war and the struggling economy, Bush's job approval rating has reached a new low for the third month straight -- at 28 percent approval, he now matches Jimmy Carter's low, surpassed only by Richard Nixon and Harry Truman.
Bush hasn't seen majority approval in 42 months, a record. And Bush disapprovers favor Obama by a whopping 68-23 percent. (Congress, at 23 percent approval, is rated even worse than Bush; it's 35 percent approval specifically for the Democrats in Congress, 25 percent for the Republicans.)
Another factor is changeability. Nearly three in 10 Americans say they could change their minds (or have no current preference).
That seems to be evidenced by night-to-night variability in preferences; in this four-night poll, Obama held a much larger lead Sunday compared with Thursday through Saturday results. This nightly variability could be one reason some recent polls have differed; e.g., Newsweek had a 15-point Obama lead in a two-day poll in mid-June, when other contemporaneous polls did not; and Newsweek had that tighten to 3 points this past weekend, again in a two-day poll.
One apparently neutral factor is the question of flip-flopping, as each candidate tries both to appeal to the center as well as to keep his base invigorated.
The bottom line is that inflexibility -- the flipside of flip-flopping -- is not in demand: Americans by a very lopsided 78-18 percent say it's more important for a candidate to "adjust his positions to changing circumstances" than to "hold positions without changing them."
McCain and Obama alike, moreover, are seen equally as having flipped-flopped on the issues (by 47 and 49 percent, respectively). And they run evenly, 43-45 percent, in ratings on who's "more consistent in his positions." On a related attribute, Obama leads McCain by 7 points as more honest and trustworthy, with sharp partisan divisions in those views.
All told, Obama leads McCain among registered voters by 8 points in this ABC/Post poll, 50-42 percent.
Turnout makes a difference: It's 51-39 percent among all adults overall, but narrows to a close 49-46 percent match among likely voters. As noted, Republicans are among those more likely to vote; young adults, less so.