This early in the election cycle, it's hard to say that Internet volume at candidate Web sites is an indicator of ultimate success at the polls. If anything, the data show that the increasing attention being placed on the Internet by candidates is a double-edged sword, since they are getting the most attention online for video clips beyond their control.

Nielsen found a far greater number of unique visitors watched Democratic candidate YouTube videos in March — 1.54 million visitors, compared with Republicans' 108,000 visitors. But that number was high because of the anti-Clinton "1984" video, which was produced by a supporter of her leading rival, Illinois Sen. Barack Obama. By April, traffic had stabilized, Nielsen found, and both sides attracted 300,000-400,000 unique viewers on YouTube.

Mrs. Clinton's Web videos drew the most attention, drawing 23.2% of the total time in April spent by YouTube visitors viewing political videos. Mr. Obama followed with 20%. Former North Carolina Sen. John Edwards was third, at 16.1%

The Republicans followed, led by Mr. McCain, who drew 14.9% of all political viewing time. His two main rivals — former Massachusetts Gov. Mitt Romney and former New York Mayor Rudy Giuliani — lagged behind at 6.2% and 1.3%, respectively.

Clips of speeches and debate performances make up the majority of the video clips being uploaded by the campaigns these days.