As and go barreling neck-and-neck into the homestretch, advisers for both men know one thing for certain: Nothing is.
More so than any presidential race in recent history, this one may be determined by forces beyond the control of either candidate.
How far will housing values fall? How far will oil prices rise? Will violence in Iraq erase the gains of the surge? Will Israel attack Iran? Will one of the Big Three automakers go bankrupt? Which neighbor will Russia attack next? Which bank will fail? Will terrorists strike the United States again?
It's impossible to predict much about this race, but here are seven things to watch as the unknowns become knowns:
1. Will Obama profit from pain?
Voters say the economy is their number one concern - and in nearly every poll Barack Obama enjoys a substantial, but not commanding, 10- to 15-point advantage on economic issues.
He's doing better than John Kerry or Al Gore did on the economy, he fares best in battleground states. A Democracy Corps survey taken during the GOP convention gave Obama an 11-point edge on the economy nationwide but a 15-point lead in swing states including Ohio, Nevada, Florida and Virginia.
But Obama hasn't been able to translate that advantage into big leads over McCain in the states hit hardest by the economic downtown. In fact, the race has tightened in Ohio, Pennsylvania, and hard-hit Michigan - despite McCain's support of unpopular free trade agreements, his less-than enthusiastic support of the housing bailout, his own profession of ignorance on economic matters and ample connections to Big Oil.
Race and class issues are probably sapping Obama's support. But he's also been hurt by nagging questions about his leadership experience as the GOP tries to shift the election from a referendum on Republican economic policies to a test of whether Obama is up to the job of president.
"The two immoveable objects in this campaign are that Bush takes the blame for the economy and that the economy favors the Democrats," says Michael Dimock, associate director of the Pew Research Center. "But it's now coming down to the question of Obama's leadership and capability . . . . McCain doesn't have to win on the economy, just mitigate its impact, and reframe the issue as one about leadership."
Added a Democratic pollster: "Don't look at the unemployment rate. The key metric is the percentage of voters who think Obama is ready to lead. So far, that's been around 50 to 58 percent. If that number stabilizes in the mid-50s, he'll win."
Then there's the "bitter" pill. The Illinois senator spent his teen years on Food Stamps, but he's had real trouble making white blue-collar Democrats believe that he feels their pain. And Republicans aren't letting voters forget his claim that working-class voters are so "bitter" they cling to God and guns.
2. Has Palin Peaked?
addition to the ticket probably exceeded her running mate's wildest expectations: McCain has turned an eight-point deficit in the Gallup daily tracking poll into a three-point lead.
But McCain's campaign has so far been able to protect Palin from any downside. Palin appeared before adoring crowds in the lower 48 last week, but she did so with the help of TelePrompters and under the protection of a journalist no-fly zone. On Sunday, the McCain campaign - facing increasing pressure - announced that Palin would have a sit-down with ABC's Charlie Gibson.
How will the Alaska governor hold up under a grilling about the future of NATO, the mortgage securitization crisis or Troopergate? Joe Lieberman is reportedly giving her a rushed tutorial on foreign policy, but the potential for embarrassment remains significant despite Palin's poise, sense of humor and innate smarts.
Some GOP analysts fret that her popularity has nowhere to go but down, as moderate women become more familiar with her staunch anti-abortion stance. And some are concerned that the conservative evangelicals who make up the party's base - so jazzed by Palin's selection - could sink back into a funk when they remember that Palin was just an appetizer while McCain remains the main course.
3. Can avoid the curse of Rick Lazio?
Palin's biggest test comes on Oct. 2 in St. Louis, when she faces Joe Biden in what is certain to be the most eagerly anticipated and probably the most-viewed veep debate ever.
Biden hopes to portray the Alaska governor as a neo-Dan Quayle, an out-of-depth amateur unfit to serve as president. But perils abound for the verbose, occasionally overbearing Biden, who must negotiate a gender minefield Rick Lazio blundered into during his disastrous debate against Hillary Rodham Clinton in 2000.
"I'd love to be a fly on the wall in Joe Biden's dressing room that night," says Michael Franc of the conservative Heritage Foundation. "They are really going to coach him to restrain himself."
4. The presidential candidates debate, too.
Until McCain picked Palin, the trio of presidential debates -- scheduled for Sept. 26 at Ole Miss, Oct. 7 in Nashville and Oct. 15 in Long Island - seemed likely to the defining moments in the fall campaign. They still are.
"The margins are so tight and voters have so many questions about both guys. The potential for a major, game-changing slip-up is huge," says Democratic consultant Jefrey Pollack.
Adds former Clinton pollster Geoff Garin: "The debates are the story this year… Voters need to take [the candidates'] temperature."
Neither candidate is exactly a master of the form. McCain does best when he's cracking collegial jokes, but he's prone to missteps and shows unattractive flashes of anger from time to time. Obama is a polished performer but sometimes comes across as condescending or professorial. He makes his own share of mistakes, including the comment -- during the recent Saddleback Forum -- that a question about when life begins was above his "pay grade." Over the weekend, Obama said his response to the question had been too flip, and that what he really meant was that he doesn't "presume to be able to answer these kinds of theological questions."
5. Will Hillary really help?
Obama needs Hillary Clinton on the trail - less to offset Palin than to deliver working-class whites who became her base during the primaries.
Exactly how much she'll be used is up in the air. Obama's people have presented Clinton with a list of places and dates. She's amenable - under two conditions. First, she refuses to be a "Sarah Palin attack dog," according to a person close to her. Second, she wants Obama campaign events to coincide with fundraisers to retire her $20 million-plus debt.
And what role will Bill Clinton play?
6. Wright back at you.
Obama complains that he didn't much in the way of economic solutions at the Republican Convention. There's something else he didn't hear much: the name of Reg. Jeremiah Wright.
That won't last.
It's possible that McCain himself will attack Obama over his longtime relationship with the firebrand former preacher, but it's far more likely that independent groups will run ads and barrage white, working-class voters with Wright-Obama emails during the homestretch.
And those same groups won't be shy about dwelling on Obama's more tenuous link to former Weather Underground radical Bill Ayers.
7. Will Mount McCain erupt?
When Democratic operatives were gaming out a race against McCain earlier this year, one thing seemed certain to work in their favor: At some point, McCain would blow a gaset and undo months of political anger management.
A lot of Democrats still think it will happen, citing high-profile McCain blow-ups like his May 2007 tussle with Texas Republican Sen. John Cornyn and noting that McCain seems to get more irascible when he's fatigued.
"The anger issue raises questions about his age - and when you get right down to it, that's Obama's greatest weapon against him," said a Democratic consultant.
By Glenn Thrush