-By Mike Shields, Mediaweek

NEW YORK When it comes to the Web, Democratic presidential candidate Sen. Barack Obama is drawing a much larger crowd and generating considerably more chatter, says a new report issued by Nielsen Online.

As of May, the Illinois senator attracted more than four times as many individual visitors to his Web site as did his Republican Sen. John McCain: 2.3 million unique users for Obama versus just 563,000 for McCain, found Nielsen (which, like Mediaweek, is owned by the Nielsen Co.). Of course, Obama's audience advantage during that month was likely driven in part by the then still heated primary race with New York Sen. Hillary Clinton, who only bowed out of the race last month.

But across the board, Obama is generating more Web activity than McCain, said Nielsen. For example, according to Nielsen's report, in May Obama's campaign ran over 105 million ad impressions versus just 8.6 million for McCain (though the Arizona Senator did run three times as many search impressions as did Obama, neither candidate is said to have committed major ad spending on the Web just yet - see this report for more information.).

Similarly, In June, Obama generated nearly twice as much "buzz volume" on blogs, said Nielsen, as the senator was referenced in 0.75 percent of consumer discussions, while McCain was mentioned in 0.39 percent of such discussions.

Of course, this Web disparity could all change as Election Day approaches. "It will be interesting to see how things shape up online as we head toward the general election," said Jon Gibs, vp, media analytics, Nielsen Online. "Now that the contest has shifted away from the primary season, McCain may gain more traction online. But it remains to be seen if his core demographic will embrace the medium the way Obama's has."