According to Nielson's online metrics analysis, BarackObama.com attracted 2.3 million unique visitors during the month, compared with McCain's 563,000.
"Barack Obama got a head start during primary season in using the web effectively to garner support, both in terms of votes and donations," said Jon Gibs, VP-media analytics for Nielsen Online. "Campaigns are no longer dabbling online -- we expect a candidate's Web presence to be an integral part of both campaigns."
Those numbers, of course, were expected -- and not just because Mr. McCain and the Republicans were placed on the back burner as the race between Mr. Obama and Sen. Hillary Clinton was reaching a boil in May.
Mr. Obama has been popular with young and tech-savvy voters. That he's had a number of celebrities and ad-industry professionals creating slick videos and other efforts of their own also goes a long way in building online buzz.
Also helping is the campaign's firm grasp of social networking. One of Facebook's founders, Chris Hughes, left the social networking juggernaut early last year to help design Mr. Obama's web presence -- in particular, My.BarackObama.com, where users can research local events, join fund-raising operations and even sign up to host "Unite for Change" parties.
Both candidates' websites include video and social-networking capabilities (though Mr. McCain's social-networking effort comes up short). Mr. Obama's campaign placed considerably more image-based online advertising impressions in May -- approximately 97,000 more according to the Nielsen report. In search sponsored links, though, Mr. McCain's campaign edged out Mr. Obama's by around 4,000 that month.
This glut of young, web-savvy supporters has served more or less as a base for Mr. Obama, and has worked well for the primaries. With that base on lock, Mr. Obama's web-dominance will be tested on other voting groups less likely to be reached by new media, such as older voters or those in lower income brackets.
The report also compiled the blog traffic of the candidates in June, and found that the most mentions of both candidates appeared on sites such as The Huffington Post and dailykos.com, traditionally left-leaning outlets, suggesting favorable returns for Mr. Obama, but not necessarily for Mr. McCain.
"Now that the contest has shifted away from the primary season, McCain may gain more traction online," said Mr. Gibs. "It remains to be seen if his core demographic will embrace the medium the way Obama's has."