With two months left, the presidential election seems likely to turn on whether Barack Obama can make himself sufficiently acceptable to millions of disgruntled Americans eager to toss Republicans from the White House if they see a safe, competent alternative.

President Bush, though not on the ballot, has overshadowed this election from the start. His approval ratings are hovering at record lows, and Democrats have done all they can to tie John McCain to him so tightly that they look like one person.

The Obama camp wants Nov. 4 to be a referendum on the Bush presidency, the closest they can come to running against the incumbent. In such elections, challengers must persuade most voters to do two things: Be open to firing the incumbent and agree the challenger is acceptable. If they succeed at both, the incumbent — or in this case, presumably, his party — loses.

With Bush so unpopular and millions of voters seemingly bent on making all Republicans pay, McCain must tackle both questions. He has tried to distance himself from Bush without infuriating the GOP base, going so far as to run a TV ad saying, "We're worse off than we were four years ago."

In his acceptance speech at the Republican convention, he cast himself as an agent of change, urging Americans to "fight with me" for a new direction. Republicans privately acknowledge it could be a tough sell to voters who see McCain as part of the Republican establishment.

That's when McCain and his surrogates urge voters to reject Obama on the second question, his fitness for the job. No matter how angry they are at Bush and the Republican Party, this argument goes, Obama is too inexperienced, unknown and unpredictable for them to gamble on.

They might vote for McCain unenthusiastically, but it's better than no vote at all.

That's why Republicans have belittled Obama's experience for months. He accomplished little in the U.S. Senate and Illinois Legislature, they say, a claim Obama disputes.

Republican vice presidential nominee Sarah Palin this week ridiculed Obama's post-college work as a community organizer in Chicago, a job she described as lacking responsibilities. Convention keynote speaker Rudy Giuliani said Obama "has never led anything," adding, "This is no time for on-the-job training."

Such attacks may be hitting a mark, as voters have shown uncertainty about Obama for months. In a USA Today-Gallup Poll conducted Aug. 21-23, before the party conventions, 55 percent of voters gave the Democratic Party a favorable rating, while 39 percent felt positive about the Republican Party, a 16-point spread. But in the latest daily tracking poll by Gallup, Obama's lead over McCain was only 4 percentage points.

Like McCain, Obama pushes hard on both parts of the two-prong question. The first is easier for him. McCain, when battling for his party's nomination, acknowledged supporting Bush's positions 90 percent of the time in the Senate. Obama never lets voters forget it.

"John McCain has a set of ideas that are identical to George Bush's," he told Ohio voters this week.

The second part — making undecided voters comfortable with the idea of him as president — is more challenging. With less than four years of Senate service, much of it spent running for president, Obama's legislative record is comparatively slim. He makes the most of it, noting his work on congressional ethics and a bipartisan effort to regulate nuclear weapons in former Soviet countries.

Obama, 47, spends more time telling voters that his judgment, proposals and ability to unite people are superior, and worthy of the Oval Office. His early opposition to the Iraq war was central to his primary victory over Hillary Rodham Clinton, and now he's turning it against McCain.

"John McCain was a cheerleader for us to go into Iraq," Obama told a crowd in Lancaster, Pa., on Thursday. "He was wrong and I was right."

Obama implores voters not to fall for Republican "scare tactics" that suggest he is unacceptably inexperienced and non-mainstream. Speaking Friday to workers in Duryea, Pa., he said he knows some voters are tempted to say to themselves, "I don't know. The guy hasn't been there that long in Washington. You know, he's got a funny name. We're not sure about him."

When a top McCain aide recently said the election would turn on personalities more than issues, Obama said, "What they're really saying is, 'We're going to try to scare people about Barack. So we're going to say that, you know, maybe he's got Muslim connections. Or we're going to say that, you know, he hangs out with radicals, or he's not patriotic.' Just making stuff up."

Perhaps the campaign's biggest mystery is something Obama did not explicitly address: What portion of voters will reject him because of his race, even if they tell friends and pollsters it's not a concern, and even if they are deeply unhappy with Bush?

If the number is large in key states, it could tip the election to McCain. Even if Obama prevails on the first question facing undecided voters, he might fall short on the second.

He seems keenly aware of the danger.

"They're trying to make you unsure about me," he told the Duryea workers. They won't be sorry, he promised, "if you give me this opportunity."

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Charles Babington covers politics for The Associated Press.

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