Separated by 16 years, two of the most famous and controversial TV ads in presidential campaign history share a remarkable set of traits. Both were launched by nominally independent groups, not by the candidates themselves. Both aired in just a few small markets, gaining widespread exposure only through news media coverage. Both were denounced as inaccurate and unfair.

And both the "Willie Horton" spot of 1988 and the 2004 campaign's initial commercial by Swift Boat Veterans for Truth slammed a Democrat from Massachusetts and helped a Republican candidate named George Bush.

The Horton ad caused a huge media stir when it ran during the fall of the race involving Vice President George H.W. Bush and Massachusetts Gov. Michael S. Dukakis, the Democratic nominee. The ad, launched by a group called the National Security Political Action Committee, showed a mug shot of a black convict named William Horton (although he was never known as "Willie," the ad referred to him that way) who had attacked an Oxon Hill, Md., couple while free under a Massachusetts prison furlough program overseen by Dukakis. The ad was intended to highlight allegations that Dukakis was soft on crime, but critics saw it as an attempt to stir up racial fears.

The commercial aired last month by a group of Vietnam-era naval veterans known as Swift Boat Veterans for Truth was also denounced by Democrats. That ad questioned Democratic nominee John F. Kerry's Vietnam service record, which has been a centerpiece of Kerry's campaign.

For academics who study campaign rhetoric and advertising, the parallels between the two commercials are striking.

"Both of these ads came from true believers," said William L. Benoit, a communications professor at the University of Missouri and the author of "Seeing Spots," which examines campaign advertising. "When you're a true believer, there's a tendency to go to extremes. A candidate probably couldn't get away with it, but he can certainly benefit from it because the backlash tends to go to the [independent groups], not the candidate."

In both the Horton and Swift boat cases, the respective Bush campaigns disclaimed responsibility, saying the ads were the work of unaffiliated groups. But in both instances, news media reports subsequently exposed ties between the official campaigns and the independent groups. In Horton's case, Bush's campaign manager, the late Lee Atwater, was at one point quoted as saying he intended to make Horton "a household name." But subsequent inquiry by the Federal Election Commission found no illegal coordination between the campaign and the independent group.

And neither of the Bush campaigns specifically repudiated the controversial spots -- although George W. Bush has called for an end to all advertising by "527" independent groups, several of which have spent millions of dollars on ads attacking his record. In fact, immediately after the Horton ad stopped airing, the Bush campaign ran its own ad about the Massachusetts prison release program using a picture of Horton, thereby reinforcing the initial commercial and the soft-on-crime message.

What is fascinating to Kenneth Goldstein, a political science professor who directs the University of Wisconsin political advertising study project, is the needle-in-a-haystack quality of the two ads -- the fact that they, of all campaign commercials, became so widely discussed.

Neither one, he points out, was viewed by very many people at first. The anti-Dukakis spot aired on cable TV in just two New England markets. The Swift boat commercial has had somewhat larger exposure, running about 700 times in three battleground states. But that is a pittance, Goldstein said, in a campaign in which more than 500,000 commercials have been run through mid-August. It is also in keeping with other famed negative campaign spots, such as Lyndon B. Johnson's 1964 "Daisy" commercial against Barry Goldwater. That commercial, which showed a little girl picking a daisy followed by a nuclear explosion, aired just once.

"It's impossible to tell which commercials are going to percolate up" and gain wider attention through the news media, Goldstein said. "You can't always tell from just looking at them which ones will make that leap. If I had to guess, I'd say that context and timing matter a great deal. It also shows you that the message matters."

It has not mattered much that the facts underlying the commercials were inaccurate, or at least distorted. As Dukakis and his defenders pointed out, the prison furlough program in Massachusetts had been started under a Republican administration and had bipartisan support under Dukakis, who ended the program in early 1988. Moreover, the federal government, under President Ronald Reagan and Vice President Bush, had its own furlough program.

The Swift boat ad was similarly flawed by several assertions, such as the suggestion, disputed by eyewitnesses, that Kerry's rescue of a crewmate in Vietnam had been routine, instead of under hostile fire, as his medal citation stated.

But truth in advertising may be less important than the political context in which the ads appeared. In 1988, the Bush campaign tried to paint Dukakis as a weak liberal -- insufficiently tough on crime, soft on the military, unpatriotic, a governor who permitted pollution of Boston Harbor to go unchecked. The Horton ad, coupled with a Bush campaign commercial showing Dukakis riding sheepishly in a tank, helped to seal this image, Benoit said.

The Swift boat ad has similarly reinforced the current Bush campaign's efforts to raise doubts about Kerry's character, especially to portray him as indecisive and untrustworthy.

The ultimate effect of the Swift boat spot may not be known until Election Day, but Kerry may escape less damaged than Dukakis because the Swift boat controversy peaked earlier than Horton, Benoit said. What's more, Sen. John McCain (R-Ariz.) has defended Kerry against the attack; no Republican came to Dukakis's aid in 1988.

Still, the Swift boat ad achieved several measures of political success. Polls show the ad raised some doubts among voters about Kerry's military service. More important, it has done what all negative political ads hope to achieve: It put Kerry on the defensive, keeping him, at least temporarily, from getting his message out about the war in Iraq, the economy and other issues. "It changed the dialogue," Benoit said. "In that sense, it has been very effective."

Even so, Kerry's relatively quick response to the Swift boat campaign may be one of the lasting legacies of Willie Horton. Dukakis and several of his aides, including some now involved in Kerry's campaign, repeatedly expressed regret for not being quicker and more forceful in reacting to the media hurricane whipped up by the Horton controversy. To combat this, Bill Clinton established his "rapid response" team in 1992 to respond immediately to negative charges leveled against him by President George H.W. Bush and his allies.

It is now standard operating procedure, for Democrats and Republicans.