He has to persuade Americans, overwhelmingly unhappy with George W. Bush's presidency, to put a Republican in the White House again.

With Americans clamoring for change, McCain has to make the case that he _ a 72-year-old senator with almost three decades in Congress _ is more likely to shake up Washington than is Obama, a 47-year-old political newcomer.

McCain's strength is national security at a time Americans are worried mostly about the economy. He is a champion of free markets at a time Americans are angry about the $700 billion bailout of the financial system.

His rival, meanwhile, is a youthful, sharp-minded, charismatic candidate who has shattered fundraising records, built an awesome political operation and inspired waves of young Americans with his message of hope.

All this suggests an Obama landslide. That is possible. Obama has a clear advantage in the state-by-state competition for electoral votes that will determine the winner.

But in the largely symbolic national vote, polls suggest the race may not be a blow out. While some give Obama a big lead, others in the final weeks show only a single-digit advantage.

And it could shrink further. An AP-Yahoo News poll released Friday found one in seven voters hasn't fully committed to either candidate.

How could it possibly be close?

The first instinct might be to point to Obama's race. Certainly some people will not vote for a black candidate. But that may be offset, at least in part, by a greater turnout from black voters, plus the support of Americans thrilled to see the country turn a page in its troubled racial history.

Then there are the smears Obama endured _ lies about his background, religion and nationality. Internet-fueled rumors falsely label him a Muslim, which, despite American pride in religious freedom, remains a pejorative among some voters.

But there are other reasons. The sharp social divide seen in the 2004 election, when Bush defeated John Kerry, has not disappeared. For many voters, Obama is simply seen as too liberal, especially on issues like taxes, abortion and gun rights.

Obama has had difficulty connecting with working-class voters. He did not help his cause in primary season when he made comments once about bitter rural voters who cling to guns and religion because of their economic frustrations. Some people were disturbed by Obama's longtime relationship with pastor Jeremiah Wright, whose anti-American diatribes circulated on the Internet.

Some voters are concerned about his relative lack of experience, especially as America is engaged in two wars and memories of the Sept. 11, 2001, attacks remain fresh.

McCain is about as formidable a candidate as Republicans could have offered, given the party's unpopularity. Unlike other Republicans, McCain could try to distance himself from Bush. Though he has agreed with Bush on most policies, he defied the administration on some big issues, including global warming, treatment of terrorism detainees and campaign finance.

He is widely admired for the 5 1/2 years he endured as a prisoner of war in Vietnam. While he supports the unpopular Iraq war, he won respect, at least among Republicans, for advocating the buildup of U.S. troops in Iraq _ a policy credited with helping to reduce violence.

Yet, given the political climate, it would be hard for a Republican to win even with a flawless campaign. And McCain's campaign has been far from flawless.

At times he has appeared erratic. As the financial crisis deepened, McCain said he would suspend his campaign to help policymakers work out a bailout package. Then he resumed campaigning before anything was resolved.

His themes kept shifting. First, he stressed his experience on national security _ then undermined the issue by selecting a vice presidential running mate with no national security credentials: Sarah Palin, the first-term governor of Alaska.

The emphasis shifted to how he and Palin were the real mavericks who would overhaul Washington. Then came the attacks on Obama, playing up his association with a 1960s-era radical. In the final weeks, his campaign cast Obama as a socialist and McCain kept talking about "Joe the Plumber," an unlicensed plumber who had challenged Obama about his tax proposals.

McCain's choice of Palin, an ardent opponent of abortion, delighted conservatives. But her poor performance in interviews raised questions about her readiness to serve as president if McCain were incapacitated. McCain, a cancer survivor, would be oldest first-term U.S. president.

At times, McCain appeared hurt by factors beyond his control.

As gasoline prices soared months ago, McCain scored points by promising to pursue offshore oil drilling. Then gas prices plummeted and the issue faded.

He talked about fighting against millions of dollars in wasteful government spending. But those figures seemed almost trivial while the government was throwing around hundreds of billions for the bailout.

McCain might have hoped that Bush would keep a low profile and not further stir anti-Republican sentiment. But the financial crisis kept Bush in the spotlight with his almost daily pronouncements on the economy.

That could only help Obama as he kept pounding on his campaign theme, linking McCain to Bush and the weak U.S. economy. And unlike McCain, Obama was consistently cool, calm and on-message.

In the campaign's final days, everything seems to be going Obama's way. McCain's campaign is struggling. Obama has more money and an incredible network of volunteers and staff to get out the vote.

Yet the ever-shifting polls suggest that an Obama victory, likely as it is, still is not guaranteed.

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Ken Guggenheim is the AP world service's Washington editor.