Even in the midst of a bare-knuckles presidential contest, the TV ads saturating the U.S. Senate race in Minnesota are among the toughest in the nation.

In one, there's Sen. Norm Coleman, introducing videos showing his opponent, Al Franken, erupting in obscenity-laced rages a few years ago. "I'm mean sometimes," Franken explains in one scene.

In another, there's a woman struggling for composure as she recalls the death of her son, Stuart, in Iraq. "I don't blame the Army for our son's death," she says. "I just blame the bad policies on President Bush, Norm Coleman, who voted for this."

With six weeks to go in the Senate race, the two leading candidates and their allies are using increasingly harsh messages to reach voters -- even as evidence mounts that their darkly calculating behavior may be chasing supporters away. They're making the risky bet that going more negative now will produce positive results in November.

The intensifying crossfire of hard-edged advertising in the close race is showing once again that such attacks remain a staple of political campaigns, despite voters' professed disgust with them.

Negative ads tend to be remembered longer than positive ones, research says. But there's a possibility of a backlash.

That's because a negative ad tells voters something about the attacker as well as the attacked.

"A negative ad, if it is successful, will lower the voter's evaluation of both candidates," said Matt Grossmann, an assistant professor of political science at Michigan State University. "It will just lower the evaluation of the candidate the ad is run against more."

At least that's the calculation, said Robert Lichter, president of the Center for Media and Public Affairs in Washington.

"It's a risk you take and a deal you make: 'My popularity goes down. If his goes down more, that's OK,' " said Lichter. "It's a race to the bottom. You just want to move to the bottom a little slower."

In fact, both candidates have seen eroding support over the last few months as the attack campaigns have intensified, with Coleman declining faster.

In May, a Star Tribune Minnesota Poll of registered voters had Coleman leading 51 percent to 44 percent. Last week the poll showed Coleman leading 41 percent to 37 percent among likely voters.

Coleman's 10-point fall, along with Franken's seven-point drop, meant that Franken closed the gap a bit -- even as both candidates shed supporters.

Independence Party nominee Dean Barkley entered the race in July, and he took 13 percent of the likely voters in the most recent survey.

Some poll respondents said attack ads influenced their decisions.

"I don't like the negative ads, I despise them," said Mike Jacobson, 53, of Hibbing, who said he supports Barkley. "It actually turned me off a little bit on Franken as a result of some of them."

"One of the things I've always liked about the independent candidate is they kind of talk about what's important rather than the other guy," Jacobson said.

Researchers hear complaints about negative ads all the time from voters in focus groups, said Grossmann.

"But when you ask them to articulate their views about the candidates, they repeat many of the criticisms that were leveled by their opponents in negative ads," Grossmann said. "They tend to learn more from the negative ads than the positive ads. They're more memorable over time."

One Franken ad opens with banjo music and a picture of Coleman next to "Indicted Senator Ted Stevens," and describes "free luxury fishing trips" Coleman took with the Alaska Republican. In another, a talking fish says special interests "have really set their hooks into Norm." And there's an ad attacking Coleman for his vote barring Medicare from negotiating lower drug prices for seniors -- omitting that he later voted to take up a bill authorizing price negotiations.

For his part, Coleman created an ad with three bowlers who chat about Franken's "foul-mouth attacks on anyone he disagrees with ... and writing all that juicy porn." A little girl tells how she doesn't pay taxes and "Al Franken doesn't ... either" -- dismissing his excuse that he paid taxes in the wrong states because of an accounting error.

Coleman offers his own Medicare ad, saying Franken "opposed prescription drugs for seniors." Franken said he would have voted against the bill that authorized the prescription program because it barred negotiating lower drug prices.

Negative ads that attack character rather than issues -- such as the one on Franken's vulgarity -- agitate viewers the most, Grossmann said. "But that doesn't mean that in the end they wouldn't remember more negative things about Franken than they do about Coleman."

Some types of negative ads appear to be more effective than others. Researchers at Arizona State University recently surveyed 700 people and found that uncivil but relevant messages produce the most negative evaluations of a targeted candidate -- more so than civil but relevant ads.

In recent days each candidate has been running ads rebutting the other's negative ads and projecting an aw-shucks demeanor. Coleman says Franken's attempts to tie the senator to special interests are no more serious than showing unattractive pictures of him.

Franken, responding to Coleman's ad on his outbursts, downplays "old clips of me in some pretty, well, passionate moments. Look, I'm not a politician, I guess I get outraged."

As the Washington Post political blog, the Fix, declared that the Minnesota race "has featured some of the hardest hitting television ads in any contest (the presidential included)," the Coleman and Franken camps held dueling media events to express righteous indignation over the other side's advertising.

Coleman supporters recycled attacks on Franken for having written a raunchy article for Playboy and his tax problems, while announcing that they had formed a "truth squad" to counter his ads.

Later, DFL spokesman John Stiles held a counter briefing where he belittled the truth squad and recited earlier attacks on Coleman for being "one of George Bush's closest allies" and a tool of oil and pharmaceutical interests in Washington.

The ads on Franken's vulgarity and tax problems promote a longstanding Republican theme that he lacks the character to be senator, while the ad linking Coleman to the death of the soldier deals with a substantive policy issue, said Jane Kirtley, Silha Professor of Media Ethics and Law at the University of Minnesota.

"It goes right to the heart of the matter if you think the war is still a central issue of this campaign, and if you hold Coleman at least partially responsible," she said. Still, the war as a campaign issue may now be overshadowed by the economy.

"I'm starting to wonder if this particular ad has already passed it's 'sell-by' date," Kirtley said. "I don't know how many viewers are reaching the point where they're basically turning off ads. That's one of the great things about the remote control, we can make them go away."