The most concerned face wins

BarackObama.com had the greatest number of unique visitors among presidential campaign websites in July, with 717,000, outpacing fellow Democrats Hillary Clinton and John Edwards, boasting 437,000 and 348,000 unique visitors respectively, according to (pdf) Nielsen/NetRatings, reports MarketingCharts.

Among Republicans, Fred Thompson's ImWithFred.com drew the most unique visitors in July, with 381,000, though he had not yet announced his candidacy, NetRatings reported.

The former Tennessee senator and "Law and Order" star, who has relied heavily on the internet to generate political exposure, announced his candidacy at midnight last night in a webcast on Fred08.com, where ImWithFred.com traffic is now directed.

Former New York mayor Rudy Giuliani ranked No. 2 among Republicans online, with 124,000 unique visitors; Mitt Romney ranked a close third, with 116,000 unique visitors;Ron Paul and John McCain rounded out the Republican campaign sites with 113,000 and 104,000 unique visitors, respectively, according to the data.

Among other Nielsen data reported:

Online Campaign Advertising

John McCain continued to lead online advertising among presidential candidates in July, with a total of 7.7 million online ad impressions. Mitt Romney took the No. 2 spot with 4.7 million impressions, while Dennis Kucinich ranked No. 3 with 820,000 impressions.

ThompsonTerritory.com, a site established to promote and sell Fred Thompson t-shirts and memorabilia, placed 3.6 million ad impressions for the month of July - well before the official Thompson announcement.

The Buzz

According to Nielsen BuzzMetrics' BlogPulse, blog buzz around the keyword "Fred Thompson" (in reference to president) has been rising in the days leading up to the expected announcement. Citation levels are up threefold versus an early August lull.

Politics on YouTube

On YouTube, Democrats are still dominating in the popularity of their politics-related video content, with 475,000 unique visitors in July, compared with the Republicans' 275,000.

But visitors to the Republican videos are staying longer, with an average visit of more than 3.5 minutes compared with an average visit of less than 1.5 minutes at the Democratic content.