By JEREMY W. PETERS

When presidential candidates have a message they want voters to hear far and wide, they have typically turned to that old campaign standby: the television ad.

But as President Obama and his advisers prepare to begin their general election push, they are turning first not to a 30-second commercial but a 17-minute online documentary that they hope will be shared and spread online through social networks and e-mail.

When the Tom Hanks-narrated, Hollywood-style documentary, called “The Road We’ve Traveled,” is set to go online Thursday night, it will appear on a new YouTube platform that enables the Obama campaign to turn the passive experience of watching a video into an organizing and fund-raising tool. The technology will allow viewers to post campaign content to their Facebook pages, volunteer and donate all without having to leave Mr. Obama’s dedicated YouTube page.

Eventually campaign strategists hope to use the new software to focus on people in highly specific ways. For example, if someone watches a video about a certain geographic location, like Florida, a list of that person’s Facebook friends in Florida would appear alongside the video with a message from the campaign that suggests recommending the video to them.

The Obama campaign’s efforts underscore the importance that political campaigns now attach to Web video and the role the medium will probably play in the coming election. Once best known in politics as the venue for viral parodies and hastily produced response efforts, online video is vital in the way campaigns communicate with and persuade voters.

“The importance of video is so new for campaigns, even relative to ’08,” said Teddy Goff, the Obama campaign’s digital director. “Now it’s in some ways the primary way our digital operation communicates with supporters. And increasingly it will be the primary way we communicate with undecided voters.”

Television is likely to remain the dominant way campaigns reach voters for the foreseeable future. Experts predict that about 10 percent of the campaigns’ advertising budgets this year will be spent on the Web. But online video offers campaigns a way to connect with people they know are engaged and not fast-forwarding through messages on their DVR players or flipping channels during commercials.

And, perhaps more important, it offers them a way to disseminate their messages into online communities where friends and family members share, discuss and debate. Campaigns believe that helps elevate their messages beyond propaganda.

“This year it’s all about getting your message into those trusted networks because everyone is suspicious about politicians,” said Darrell West, of the Center for Technology Innovation at the Brookings Institution. “It’s hard to be persuasive through a direct advertisement. But if you can get people to share videos, it adds a degree of credibility because a friend is endorsing it. People will take it more seriously.”

“The Road We’ve Traveled” was conceived and produced by the campaign to stand out from standard political video fare. For starters, it was directed by Davis Guggenheim, the Oscar-winning director whose film credits include “Waiting for Superman” and “An Inconvenient Truth.” It features interviews with Obama administration officials past and present, including Vice President Joseph R. Biden Jr. and Rahm Emanuel, the former White House chief of staff. Former President Bill Clinton makes an appearance as well, hailing Mr. Obama’s decision to kill Osama bin Laden as one he hoped he would have had the courage to make as president.

The new YouTube platform that the Obama campaign will use to release the documentary gives anyone visiting the president’s YouTube page a number of options to share the content or pledge support, the kind of one-click approach that campaigns now see as an integral part of their digital strategies.

Mitt Romney’s campaign is using similar technology with its Web videos. Visitors can donate, volunteer and share content, all within Mr. Romney’s YouTube page. The campaign has worked to keep its video offerings dynamic, producing roughly two a week over the course of the campaign.

The Obama campaign has taken a similar one-stop-shopping approach to streamline online donations. Borrowing a technique from online merchants like Amazon and Fresh Direct, repeat donors do not need to resubmit their credit card information to make a pledge. The campaign saves it on file, and all the donor has to do is click.

Where online video offers some of the most potential, strategists say, is in modernizing the traditional aspects of campaigning, like get-out-the-vote efforts and responses to attacks from opponents.

“It’s the ability to get your message out quickly that makes all the difference,” said Zac Moffatt, the Romney campaign’s digital director. “And that’s really where I think YouTube has found a niche in politics.”

Some strategists said what has changed in this election is the ability to turn Web video into something people act on. “One of the biggest challenges with YouTube is giving people a clear action to take after viewing,” said Stephen Muller, the Obama campaign’s video director. “The goal is to bring our engagement tools to our supporters.”