Democratic presidential candidates are trumping Republicans in terms of visitor traffic and time spent on their Web sites, according to new Nielsen//NetRatings data for July. But the Republicans are spending more for online advertising--specifically sponsored links--with more than double the impressions of the leading Democrats combined.

Sen. Barack Obama led both the Democrat and Republican packs when it came to traffic, with some 717,000 unique visitors to BarackObama.com last month. Hillary Clinton and John Edwards took second and third place with 437,000 and 348,000 uniques, respectively.

With the average visitor spending over 8 minutes at HillaryClinton.com, Senator Clinton was the top Democrat in terms of user engagement. She edged out Obama's 7:53 minutes. In contrast, visitors to JohnEdwards.com typically spent just 3:43 engaged with his campaign and policy information.

The top Republican site in July was Fred Thompson's ImWithFred.com--drawing 381,000 unique visitors, despite the fact that the former Tennessee senator and "Law & Order" star hadn't even announced his candidacy. (He has since made his intention to run official, in a Webcast this week at Fred08.com--where traffic from ImWithFred.com is now being directed.) Meanwhile, Republicans Rudy Giuliani and Mitt Romney had a close battle for second and third place, with 124,000 and 116,000 unique visitors.

According to political analyst and blogger Colin Delany, Thompson's high pre-announcement numbers point to a Republican population that was dissatisfied with their official candidates at the time, as they were actively exploring alternatives.

Levels of engagement on the three most-trafficked sites ran the gamut from high--with Rudy Giuliani's JoinRudy2008.com keeping visitors for 7:33 minutes--to an abysmal average of 49 seconds at MittRomney.com. Curiosity may have drawn users to Thompson's site, but he had a ways to go with engagement. Visitors stayed for about 1:34 minutes.

While site traffic is one measure of how excited people are about a candidate, the numbers don't reveal whether they're converting that interest into solid support, Delany tells Online Media Daily.

"Hillary outpolls Obama, for example. But the traffic shows that his supporters are really interested in finding out more," Delany says. "As far as engagement and repeat visits, Obama's site also goes out of its way to build community, encouraging users to set up profiles and fund-raise. But the campaigns aren't going to tell us how many people sign up for their email lists, and can't tell us whether supporters will actually get out and vote--and that's the kicker."

The Republican candidates are spending more to advertise on the Web, according to Nielsen//NetRatings AdRelevance. John McCain and Mitt Romney (the top two) paid for nearly 7.7 million and over 4.6 million in sponsored link impressions, respectively. In comparison, Democrats Clinton and Dennis Kucinich together didn't even account for half of either Republican's sponsored link spending--with 327,000 and 820,000 impressions, respectively.

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