Obama didn't make clear what distinctions McCain was likely to raise regarding the presidents on U.S. currency, men who are white and, for the most part, much older than Obama when they were elected. McCain has not raised Obama's race as an issue in the campaign, though he has said that Obama lacks experience.
Stumping in an economically challenged battleground state, Obama argued Wednesday that President Bush and McCain will resort to scare tactics to maintain their hold on the White House because they have little else to offer voters.
"Nobody thinks that Bush and McCain have a real answer to the challenges we face. So what they're going to try to do is make you scared of me," Obama said. "You know, he's not patriotic enough, he's got a funny name, you know, he doesn't look like all those other presidents on the dollar bills."
McCain spokesman Tucker Bounds declined to comment Thursday on Obama's remarks.
Obama spokesman Robert Gibbs said the senator was not referring to race.
"What Barack Obama was talking about was that he didn't get here after spending decades in Washington," Gibbs said Thursday. "There is nothing more to this than the fact that he was describing that he was new to the political scene. He was referring to the fact that he didn't come into the race with the history of others. It is not about race."
Obama often makes references to his distinctions as a candidate, such as saying there are doubts among some voters because he has "a funny name." At times he refers to his race as well, saying he looks different that any previous candidate but then adding that the differences are not just about race. Addressing supporters Tuesday night at a fundraiser in Springfield, he said, "It's a leap, electing a 46-year-old black guy named Barack Obama."
During a round of appearances in Missouri, Obama worked to link McCain to the unpopular Bush, saying the Republican senator from Arizona would serve the equivalent of a third Bush term if elected. He said the country can't afford more of the same and expects different results.
"That's a definition of madness, but that's what John McCain is offering. He's offering Bush economic policies and Karl Rove politics," Obama said.
He pressed the theme later at a rain-soaked barbecue in Union, Mo.
"They're going to say I'm a risky guy," Obama said. "What they're going to argue is I'm too risky. The real risk is that we miss the moment, that we do not do what's needed because we're afraid."
For its part, McCain's campaign on Wednesday released a withering television ad comparing Obama to Britney Spears and Paris Hilton, suggesting the Democrat is little more than a vapid but widely recognized media concoction.
"He's the biggest celebrity in the world, but is he ready to lead?" the voiceover asks in the ad, which mixes images of Obama on his trip to Europe last week with video of the 20-something pop stars.
Obama's campaign quickly responded with a commercial of its own, dismissing McCain's complaints as "baloney" and "baseless."
Throughout the day, Obama argued that McCain "thinks we're on the right track," drawing boos from his audiences.
"These anxieties seem to be growing with each passing day," Obama said. "We can either choose a new direction for our economy or we can keep doing what we've been doing. My opponent, John McCain, thinks we're on the right track."
That elicited boos from some of the 1,500 people who filled a Springfield high school gymnasium. When an AP-Ipsos poll asked the "right track, wrong track" question this month, 77 percent said they thought the country was on the wrong track. The same poll set Bush's approval rating at 28 percent. Both were records for the AP-Ipsos survey.
"It's true that change is hard, change isn't easy," Obama said. "Nobody here thinks that Bush or McCain has a real answer for the challenges we face so what they're going to try to do is make you scared about me."
Obama also compared himself to western legend Wild Bill Hickok, who he said had fought a duel in Springfield.
"I'm ready to duel John McCain on taxes right here, quick draw," Obama said, prompting a quick retort from the other side.
"If Barack Obama wants this so-called duel then why did he and his entourage run for the hills when John McCain challenged him to 10 town halls?" asked Bounds, the McCain spokesman.
Obama responded after shaking hands at a restaurant in Lebanon, Mo.
"I don't hear very much positive from Sen. McCain," he said. "He seems to be only talking about me. You need to ask John McCain what he's for, not just what he's against."