The Obama campaign appeared to ramp up online efforts in May and June as ad impressions surged to 150 million and 244 million, respectively, while McCain's fell to a low of 3.2 million in June.

Obama also outstripped McCain in the average number of monthly visitors to their respective campaign sites (2.2 million to 583,000) and searches of "Obama" versus "McCain" (5.3 million to 1.3 million).

"Not only have the two campaigns placed a different level of emphasis on the importance of using online advertising as part of the media mix, but their execution also shows stark differences," said Andrew Lipsman, senior analyst at comScore, in a statement.

"While Obama's ads tend to be 'brand-building' ads encouraging people to join the movement, McCain's ads are often issue-oriented. Additionally, while Obama ads have an almost universally positive message, McCain ads feature a mix of positive and negative messages."

One area where McCain turned the tables was video. His site had more than triple the video views of Obama's at 2.1 million to 612,000. ComScore attributed the Republican presidential nominee's online upset to video being featured more prominently on the home page of JohnMcCain.com than on BarackObama.com.

McCain has also been closing the gap more recently in video views on YouTube, where Obama has far more ads and original shorts. In the last month he had 6.5 million views to Obama's 9.8 million, according to video tracking site TubeMogul. McCain gained from the barrage of TV ads (posted online) he began running against Obama in July.

The McCain campaign has begun to generate more buzz online in the last week in particular with the vice presidential pick of Alaska Governor Sarah Palin. Among the presidential candidates, blog mentions of McCain jumped 140% to nearly 7 million compared to a 36% drop to 4 million for Obama for the week ending Sept. 4, according to Technorati.

For the week ending Aug. 31 -- two days after the announcement of Palin's selection -- traffic to McCain's site more than tripled to 1.8 million, according to Nielsen Online. Obama's increased by one-third to 3.4 million.

While Obama continues to enjoy sizeable leads in other social media metrics like Facebook supporters and MySpace friends, McCain is now growing faster in those benchmarks as well. Over the last week, his support on Facebook increased 19% to Obama's 16.5%, and on MySpace by 14.8% to Obama's 6.2%.

So what does it all add up to? Obama leads McCain 48.8% to 42.8% in the presidential race, according to an average of national polls compiled by the site Real Clear Politics. Like Obama following the Democratic National Convention, McCain is likely to get a polling bounce coming out of the Republican convention this week, which could tighten things up even more.

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