In a dim Culver City editing room, two video snippets of Republican presidential hopeful John McCain fill the monitors. In the first, he says same-sex marriage should be allowed. In the second, he says it should be illegal.

The clips are part of the payoff of a weeks-long hunt by filmmaker Robert Greenwald and his production team for damaging Internet video of the Arizona senator.

Greenwald, the producerdirector of scathing documentaries about Fox News and Wal-Mart, hopes to shatter McCain’s image as a straight-talking maverick. But instead of creating a full-length film, he is assembling clips of McCain for a series of two-minute Web videos. The idea is to turn McCain’s own words against him, spreading the videos through e-mail, blogs and websites.

“The effectiveness is hearing and seeing him say stuff,” Greenwald said in the editing bay. The videos “go right to the character issue — who he is.”

The first whack at McCain, now on the video-sharing site YouTube, joins a rapidly growing collection of Web videos posted by critics of leading contenders in the 2008 presidential race. Targets so far include Barack Obama, Rudolph W. Giuliani, John Edwards, Mitt Romney and Hillary Rodham Clinton.

The explosion of video-sharing on the Web poses major risks for presidential candidates: Gaffes and inconsistent statements witnessed by dozens can be e-mailed instantly to millions.

The White House ambitions of Republican George Allen of Virginia were dashed in no small part by a Web video that showed him, at a campaign event, calling an Indian American “macaca.” Allen also lost his November bid for reelection to the Senate.

And Romney, a former Massachusetts governor, was hit this month with an anonymously posted YouTube video made of footage from a 1994 debate in which he took liberal stands on abortion and other matters. Romney, who has staked out more conservative positions in his quest for the Republican presidential nomination, posted his own video to explain the shift.

“I was wrong on some issues back then,” he told viewers. “I’m not embarrassed to admit that.”

For the candidates, as well as their detractors, the chief attribute of Web video is its broad reach, accomplished at little or no expense.

“You can grab it, send it, link it, and at zero cost,” said Matthew Dowd, a top strategist for President Bush’s 2004 reelection campaign. “Two hundred thousand people could see it in 24 hours.”

Several White House contenders have already made promotional Web videos a central part of their communications strategy, using them to reach supporters directly, without a media filter. Democrats Clinton, Edwards, Obama and New Mexico Gov. Bill Richardson each made Web video statements for their campaign launches.

Clinton has been especially aggressive. The New York senator and presumed party front-runner took questions from supporters on three evenings last week in half-hour Web chats. As part of a broader effort to warm up voters (a fireplace crackled in the background when she appeared Wednesday on NBC’s “Today” show), Clinton pivoted from Iraq and healthcare to the delights of gardening, dog-walking, movie watching, swimming and even closet cleaning.

McCain is planning his own Web version of reality TV. He has hired a videographer to record behind-the-scenes campaign moments of the senator in relaxed settings.

“What the campaign can do in a Web video is show a more personal side of the candidate,” said Spencer Whelan, who works on McCain’s online communications team.

But the same technology allows others to broadcast — often anonymously — videos utterly outside the campaigns’ control. Already, attack videos range from the caustic to the ridiculous.

McCain’s comic potential is on display in a YouTube video featuring the melodically impaired senator singing lines from “The Way We Were” and other Barbra Streisand tunes in a “Saturday Night Live” skit.

Another video on the site shows Giuliani dressed in drag, with Donald Trump nestling his face in the former New York City mayor’s fake breasts — a gag from a long-ago press dinner that struck many New Yorkers as funny, but might puzzle some Republican primary voters in, say, South Carolina.

Edwards, the 2004 Democratic nominee for vice president, is the subject of a popular prank video that uses humor to skewer the former North Carolina senator. Mocked by critics as “the Breck girl” in 2004, the telegenic candidate is shown fussing with his hair for a full two minutes in preparation for a TV interview, as Julie Andrews sings “I Feel Pretty.” YouTube visitors have viewed it more than 27,000 times.

Among the Clinton material posted on the site is a home-video excerpt, first broadcast by ABC News, that shows her confiding to someone at a campaign fundraiser that she avoided e-mail because of constant investigations of the White House during her husband’s presidency.

Obama, a newcomer to presidential politics, is just starting to draw the sort of negative attention that the Clintons have long attracted. Last week, Chicago-area political consultant Joe Novak posted several Web videos taking aim at the Illinois senator’s wife, Michelle, for her healthcare business dealings.

“I’ve gotten very angry over the fawning cheerleading that’s going on in this city by so-called reporters,” Novak said.

Obama campaign spokesman Dan Pfeiffer said Novak’s videos show that the Web “is rapidly becoming the place to put video that is too inaccurate and too scurrilous to put on television.”