MoveOn.org, a grassroots advocacy group, broadcast this anti-Bush ad in July 2003: "George Bush told us Iraq was a nuclear threat. He said they were trying to purchase uranium, that they were rebuilding their nuclear facilities. So we went to war. Now there's evidence we were misled, and almost every day, Americans are dying in Iraq. We need the truth, not a cover-up."
Now other organizations are coming out with ads of their own attacking Kerry.
One is sponsored by a group called the Swift Boat Veterans for Truth, named after the U.S. Navy river vessels used by Kerry and other soldiers in Vietnam.
The ad includes testimony from five Vietnam veterans who say Kerry did not deserve his medals from that war. They also accuse him of dishonoring his country by protesting the Vietnam conflict when he returned home: "John Kerry is no war hero." "He betrayed all his shipmates, he lied before the Senate." "John Kerry betrayed the men and women he served with in Vietnam." "He dishonored his country, he most certainly did." "I served with John Kerry. John Kerry cannot be trusted."
The Swift Boat ad sparked an angry response from many observers.
Republican Senator John McCain, a fellow Vietnam veteran, called the attack dishonorable and dishonest.
Even the Bush administration attempted to distance itself from the ad. White House spokesperson Scott McClellan said: "We have not and we will not question Senator Kerry's service in Vietnam."
Even one of the Swift Boat members himself has appeared to express doubt about his criticism of Kerry. But the Swift Boat ad has served to highlight the wider issue of negative advertising and its role in American politics.
Bill Frenzel, who served for two decades as a member of the House of Representatives, says there is a difference between criticizing a candidate's political performance -- as the MoveOn.org ad does -- and launching a harsh personal attack like the Swift Boat ad.
"Most officeholders feel that if anybody says anything about their record that's bad, it's negative [advertising]. But it isn't. That is the only way you can attack incumbents, by saying, 'Hey, this guy hasn't done a good job and here's where he went wrong.' That clearly is not a negative [ad]. But if you say about John Kerry, 'Well, he won these medals, but he shouldn't have won them,' I think most people would agree that that's negative advertising," Frenzel says.
By law, nonprofit advocacy groups like Swift Boat Veterans for Truth and MoveOn.org have the right to raise money and air their political views -- but only if they avoid contact with any political party.
Frenzel says he believes such groups keep their distance from the Kerry and Bush campaigns, despite the clear favoritism of their ads.
"I think both sides go to great pains not to be associated with -- and not to communicate with -- the campaign organizations of the candidates they are in favor of. It seems to me that they're giving the campaigns a wide berth, that they are going to great lengths to show that they are not in any way associated with the campaigns," Frenzel says.
Larry Sabato, a political analyst at the University of Virginia in Charlottesville, is not so sure. He tells RFE/RL that illicit communication is possible between a candidate and a well-financed group determined to help defeat his opponent.
So will MoveOn.org and Swift Boat Veterans for Truth have an impact on the November vote? Sabato says in this year's close race, the role of individual ads can be difficult to measure:
"This is a race that is being clearly defined on the big issues, on war and peace, on the economy. And it's very difficult for a single negative ad -- no matter how widely aired -- to really have much of an impact when the election plate is so full of big, meaty issues," Sabato says.
Is this wealth of issues the reason Kerry and Bush are not running negative ads themselves? No, Sabato says. They are staying away from negative ads for two reasons only.
"First of all, somebody else is doing it. Second, it's just too vicious, and there could be a direct backlash on the campaign. They're a little worried about it. On the other hand, they're very happy these groups are doing it. They're concerned that maybe they're going too far, but they love to see the activists in their party get some red meat," Sabato says.