Barack Obama faces an untested set of hurdles

They worry more, they say, about other issues: Will swing voters view him as too young? Too inexperienced? Or too liberal?

"I am sure there are people in Missouri that won't vote for Barack Obama because he's black, but there are not that many of them," said Sen. Claire McCaskill (D-Mo.), a swing-state leader who endorsed Obama early. "I don't think that's going be a deal breaker."

Instead, she said, Obama's most important test should he lock up the nomination will come from Republican efforts to paint him as an elitist, a social and cultural liberal outside the mainstream of American life. "The key is going to be whether Barack can avoid getting on defense on social 'wedge' issues and can stay on the offense on economic issues," McCaskill said.

Polls suggest McCaskill may be right.

A survey released this month by the independent Pew Research Center found that most voters described Arizona Sen. John McCain, the presumptive Republican nominee, as "a centrist whose views are fairly close to their own," even though McCain describes himself as a thoroughgoing conservative. The same voters described Obama as the most liberal of the candidates still in the race, well to the left of what they saw as the midpoint of American politics.

And Obama ranked below both McCain and New York Sen. Hillary Rodham Clinton, his rival for the Democratic nomination, on the question of whether the candidate was "tough enough" to protect the nation's security.

Obama has "handicaps and potential problems, race being one of them, [but] it's not the only one," Pew Center President Andrew Kohut said. "He is perceived as a liberal. He is perceived by many voters as not well grounded on foreign policy and not tough enough . . . and he has a potential problem, distinct from race, of being seen as an elitist, an intellectual."

Taken together, that's a formidable catalog of vulnerabilities. In an ordinary election year, and with a more traditional candidate, it might make a Democratic victory hard to envision. But 2008 is not an ordinary political year: Republicans are weighted down by an unpopular incumbent, a presumed nominee who supports an unpopular war, and economic troubles that threaten voters in their homes, their jobs and their pocketbooks.