All eight contenders for the Democratic presidential nomination have taken part in the first online-only debate.
The leading Democratic candidates contributed to the online debate
As well as questions on Iraq, education and healthcare posted online by voters, the candidates faced "wild card" questions from comedian Bill Maher.
Among the issues raised were legalising cannabis and taking lobbyists' money.
Hosts Yahoo and websites Slate and the Huffington Post say the so-called "mashup" is a first because viewers can pick and choose which clips they watch.
This means they can tailor the debate to their specific interests and line up clips of candidates so they can directly compare them on the same issue.
Discussions to set up a similar online forum with the Republican candidates are ongoing.
The questions for the Democratic candidates were posed via satellite link-up by TV talk-show host and debate moderator Charlie Rose.
The three topics - Iraq, healthcare and education - were chosen in an online poll on Yahoo in which more than 100,000 people voted, the Huffington Post said.
Ariana Huffington, founder of the Huffington Post, told the BBC News website that the aim of the online format was to engage web users, particularly young people, who would not sit through a 90-minute televised debate.
"It is created for those people who live online, who want to be able to control the way they interact with the material, who they want to watch on which issue," she said.
"We give a whole new audience the chance to interact with the candidates."
Questioned on the Iraq war, all the candidates agreed that this week's report to Congress by Gen David Petraeus and the US envoy to Baghdad, Ryan Crocker, had failed to convince them of the chances of political reconciliation in Iraq.
Chris Dodd said he favoured legalising cannabis for medical use
Frontrunner Hillary Clinton announced that she would release her plan to offer universal health care next week, but gave few details.
In one lively moment, she attacked her main rivals' criticism of her acceptance of campaign money from lobbyists.
"I think it's a little inauthentic for people to say 'I don't take money from lobbyists' but it's okay to take it from their spouses, their children, their associates and from people that work for companies that employ them.
"That is, you know, to me, kind of an artificial distinction."
Barack Obama and John Edwards have both pointed to Mrs Clinton's willingness to take contributions from lobbyists as a sign that she is too close to the Washington establishment.
Neither Mr Obama nor Mr Edwards accepts contributions directly from federal lobbyists but both take money from firms with lobbying businesses and from networks of friends and colleagues of lobbyists.
Asked about the role of fundraising in presidential campaigning, Mr Obama described money as "the original sin of politics" but said "some sinning" was necessary to compete.
In one of the more offbeat questions, comedian Bill Maher asked Chris Dodd whether he would legalise cannabis purposes.
Mr Dodd replied that he supports making it legal for medical use and would favour decriminalising it in order to ease pressure on overcrowded prisons.
The all-online debate is the latest in a series of experimental formats to be trialled during this election cycle.
A debate held by YouTube and CNN in July, in which video questions submitted online were put to Democratic candidates in a live discussion, was also hailed as a breakthrough moment in presidential debates.
The Republican version of the event is scheduled to take place in Florida on 28 November.