WASHINGTON (CNN) -- It was one of the most talked about moments in the 2006 campaign: "Lets give a welcome to Macaca, here.  Welcome to America and the real world of Virginia." 

Former Sen. George Allen, R-Virginia, knows firsthand the effect YouTube has had on politics. 

That was then-Virginia Republican Sen. George Allen's controversial remark to a young campaign aide working for Allen's opponent, Democrat Jim Webb.  The comment was caught on camera by the Webb aide, and was put on YouTube.  It became a smash hit. 

Allen was expected to cruise to re-election, but thanks in part to the YouTube video, Allen lost his seat by just a few thousand votes to Webb.  His loss, along with the razor thin defeat of Sen. Conrad Burns, R-Montana -- who had his own damaging moments on YouTube -- helped swing control of the Senate to the Democrats. 

Gotcha moments on YouTube, unauthorized campaign videos and hard-hitting debate questions from YouTube users are changing the political landscape.  The YouTube "Macaca moment" represents a broad new challenge for candidates, but speaks to the age-old problem of how to control the message. 

"I think it really breaks down some of the traditional barriers we have seen in American politics," Steve Grove, head of news and politics at YouTube, told CNN's John King Monday. " Time was, if you wanted to engage in a primary debate process, you had to be in New Hampshire or be in Iowa." 

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Grove said that CNN-YouTube's Democratic debate next week in Charleston, South Carolina, is a game-changer.  The user-generated questions can touch any topic and come from anywhere in the world. 

"Candidates do hate, genuinely hate, audience participation, because they like to control the environment," said longtime television news anchor Dan Rather, now the global correspondent for HDNet.  Tell candidates people will ask them questions via a YouTube video, he says, and "they get the shivers." 

It's those unscripted moments that have taken on a new life on the video-sharing Web site. 

For Allen, that short YouTube clip, viewed millions of times, had far-reaching repercussions for his political career.  Allen also lost any immediate chance at making a run for the White House. 

"If not for YouTube, Allen would most likely be one of the front-runners today for the GOP presidential nomination," says CNN Political Editor Mark Preston. 

When it comes to YouTube moments, the presidential campaign cycle is picking up where the midterm elections left off.  While candidates try to avoid a "Macaca moment," the competing campaigns are busy dispatching video "trackers" to catch one. 

A local stop on the campaign trail is suddenly a national story. 

Sen. John McCain had a YouTube moment in April. " Remember that old Beach Boys song, bomb Iran.  Bomb.  Bomb.  Bomb."  That was the Republican presidential candidate having some fun with a friendly crowd in South Carolina.  But his comments got played over and over again on YouTube and became a story picked up by the mainstream media. 

"You cannot go to a 7-Eleven or a Dunkin' Donuts unless you have a slight Indian accent.  I'm not joking," said Democratic presidential hopeful Joe Biden last July.  The senator from Delaware was joking, but his comments made the rounds on YouTube, and Biden needed to clarify just what he was talking about. 

YouTube is forcing candidates to deal head-on with their past.  The ease at which video can be posted and distributed on YouTube is giving old debate clips new life, forcing presidential hopefuls to explain conflicting positions. 

"I believe that abortion should be safe and legal."  That was former Massachusetts Gov. Mitt Romney from a 1994 debate with Sen. Ted Kennedy.  Romney has changed his position on abortion, but those old clips supported his critics' charge that Romney flip-flopped on abortion. 

Former senator and probable GOP presidential candidate Fred Thompson is facing a similar situation.  When Thompson was first running for the Senate in 1994, he was asked during a debate whether he would support or oppose laws that prohibit abortions for convenience.  Thompson responded by saying, "I don't believe that the federal government should be involved in that process."  That video is now making the rounds on YouTube and probably won't help Thompson's push to court social conservatives. 

"Is YouTube having an impact?  Of course," says Amy Walter, editor-in-chief of the National Journal's Political Hotline and a CNN Political Analyst, "but to me what's more interesting is from the strategist point of view -- which is when to react and when not to react to YouTube." 

There is a flip side to the YouTube phenomenon.  It can also help campaigns looking for new ways to harness the power of the popular Web site. 

In April, YouTube began spotlighting one campaign each week, allowing each candidate to ask anything they want. 

"What do you think our campaign song should be?"  asked Sen. Hillary Clinton, the Democratic presidential front-runner. 

YouTube users can talk back, or in the case of some Clinton supporters and opponents, sing back. 

Sometimes the campaigns are getting help without asking. 

"I got a crush on Obama" sings a young woman on a music video that was an instant hit on the site.  Sen. Barack Obama's campaign wasn't behind the video, but the buzz probably didn't hurt the Illinois senator's presidential bid. 

Then there was the "Vote Different" Orwellian clip, uploaded anonymously on YouTube, which portrayed Clinton as Big Brother in a remake of an Apple Computer ad from 1984.  Viewed millions of times on YouTube, the final frames pointed to Obama's presidential Web site. 

Those are two perfect examples of how YouTube is empowering average Americans to affect the political process like never before.  That in turn is affecting the campaigns.  The candidates don't have total control over their message any more, and that's forcing them to change the way they campaign.  E-mail to a friend

CNN Deputy Political Director Paul Steinhauser contributed to this story. 

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