But the most popular participant was not a candidate.

Comedian Bill Maher, who asked one of four questions posed to each of the eight candidates, attracted viewers 42% of the time. He quizzed the hopefuls about the Ten Commandments, marijuana legalization, the relative dangers of sugar, coal dust and terrorism, and the climate-changing impact of cows.

Yahoo, HuffingtonPost.com and Slate.com conceived the format as a way to give online viewers the ability to build a debate with video blocks of each candidate answering different questions on education, health care and the Iraq war from Public Broadcasting Service host Charlie Rose. A "wild-card" question came from Maher.

The debate was taped two weeks ago and the three Internet sites posted the video on Sept. 13. Viewers can choose the candidates they want to hear from, match them against a rival, ignore those who do not interest them and compare and contrast.

"We started off doing this as a public service," said Neeraj Khemlani, vice president of programming at Yahoo and producer of the debate. "It was in my mind, 'Let's go and try to help undecided voters."'

As of this weekend, 1.1 million people had clicked on the debates. Of those, 429,000 were between the ages of 18 and 34, according to data compiled by Yahoo.

Passive television watching is still the preferred method for most debate viewers. An Aug. 19 Democratic debate on ABC's "This Week," for instance, attracted an audience of more than 2.8 million.

But organizers of the online debate say its audience is more engaged and that the format puts the content in the viewers' hands.

On average, each mashup viewer watched 4.4 video streams for a total of seven minutes, an unusual amount of time on the Internet, where clips on the YouTube video-sharing site typically run two minutes or less.

The most viewed candidate? Sen. Hillary Rodham Clinton of New York. Of all the video clips, viewers watched 35% of hers. Sen. Barack Obama of Illinois was next with 25%, followed by former North Carolina Sen. John Edwards at 13%.

After Maher's questions, the subject of Iraq received the most attention — 35% of the clips watched addressed the war.

Heath care was next with 15%, and education followed with 9%. The health care numbers could increase because Clinton released her plan for universal health care last week, potentially driving up interest in the subject.

The Yahoo data shows that women were more interested in education and health care than men, who were more taken with Maher's questions.

Audiences age 35 and older were more interested in health care and Iraq, and those 35 and under focused on education and the wild-card question.

In Iowa, which is scheduled to hold the first contest of the presidential election season, viewers showed the most interest in Iraq. Forty-one percent of the video clips watched in the state dealt with the war.