The Internet was a critical factor in Barack Obama’s defeat of Hillary Clinton. But it could also jeopardize his chances against John McCain in the fall.

Obama has been called “the hottest start-up” in Silicon Valley and “the first real wiki-candidate.” His mastery of the Web — raising money, mobilizing voters, distributing videos — helps explain why he overcame Clinton’s enormous advantage in connections and credibility.

The Internet is value neutral, however. It delivers bad news as well as good news. Any of Obama’s unguarded or inconsistent statements are immediately scrutinized, and carefully compared to every other statement he has ever made, neatly catalogued on YouTube.

More seriously, Obama has been plagued by a swirl of untrue rumors — that he’s a Muslim and a foreigner, a radical and a racist. And his campaign admits those rumors, fueled and fast-forwarded on the Internet, pose serious threats to his candidacy.

“This campaign is only possible because of the Internet, because of the technology, because we could raise a couple of hundred million dollars (from) 1.5 million Americans, ...” Jim Margolis, Obama’s media adviser, told the Washington Post. “On the other hand, you’re constantly dealing with the misinformation that can spread quickly. ...”

McCain raised money over the Internet in 2000, and Howard Dean used social-networking sites to connect his supporters four years later. But they both lost. Obama is the first candidate to grasp the Web’s full potential as a political tool.

Start with fundraising. The astounding totals have enabled the Democrat to reject public funds (and spending limits) and finance advertising campaigns in marginal states like Georgia and Colorado months before the balloting.

The essential nature of the Internet — what makes it so different and so powerful — is that it allows immediate, interactive conversations. And that has enabled Obama to elicit far more than dollars from his donors. He’s also harnessed their energy and enthusiasm, their contacts and creativity.

The Pew Research Center reports that 46 percent of all Americans have used the Web this year to participate in the campaign. And Obama is trouncing McCain: His MySpace page lists 401,000 friends, compared to McCain’s 56,000; his videos on YouTube have been downloaded 53.4 million times, compared to McCain’s 3.7 million hits.

But the digital age has a clear downside for Obama as well. He is still an inexperienced candidate, prone to mistakes, and nothing he says goes unrecorded or unpublicized. His famous comment about “bitter” voters who “cling” to their guns and prayers will not go away. He might have disowned his former pastor, the Rev. Jeremiah Wright, but videos of Wright’s more incendiary sermons will not go away, either.

Obama ran against Clinton as an orthodox liberal, but as he tries to reposition himself as a centrist, past statements can come back to haunt him.

His biggest problem is the Internet rumor mill. If Obama supporters face lower barriers to participating in politics, so do his enemies. The New York Times reports that one popular anti-Obama video, which attacks him for having “not one, not two, but three Islamic names,” was made for $50.

In a dispatch from Ohio, the Times states that such slurs, “born on the Internet,” have “built enough word-of-mouth credibility to harden into an alternative biography.” Team Obama is so alarmed they’ve created a new Web site, “Fight the Smears,” to confront the rumors head-on. “We will aggressively push back with the truth,” said one Obama adviser.

But will that be enough? In the war of the Web worlds, which side will win? Obama’s friends or foes?

(Steve and Cokie Roberts can be contacted by e-mail at stevecokie@gmail.com.)