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When a Republican-funded group of Vietnam veterans sought to run a blistering television ad accusing John Kerry (search) of lying about his decorated war record, Democrats quickly fired off a letter to broadcasters imploring them not to air the "inflammatory, outrageous lie."

The goal: to get as many stations as possible to reject the ad and stymie potential damage from it.

It's a dance that happens often in political advertising. Republicans and Democrats try to get broadcasters to block each other's commercials by providing evidence countering claims in the spots. Sometimes they succeed and ads get pulled or changed. More often they don't and commercials are run even with questionable material.

Unlike ads by outside groups, candidates can say whatever they want to in ads and stations must run them.

That's the case with a new spot Kerry rolled out that says Bush's campaign "supports a front group attacking John Kerry's (search) military record," a reference to a group calling itself Swift Boat Veterans for Truth (search). Bush's campaign complained that the Kerry ad contained a "false and libelous charge." Stations didn't have to run the veterans' ad; they do have to run the Kerry campaign's ad responding to it, despite the Bush campaign's objections.

Broadcasters have a right to turn down other political ads that don't meet truth-telling standards followed by commercial advertisers like Pepsi, Toyota and Nike. Stations leave themselves open to lawsuits if non-candidate political ads contain potentially libelous content.

"It's basically up to the individual broadcaster to decide whether that third-party advocacy ad is appropriate for their audience," said Dennis Wharton, a National Association of Broadcasters (search) spokesman.

Still, stations rarely reject commercials — even ones with fuzzy claims. They have little incentive to: they don't get paid for ads that don't run, and they very rarely are sued.

"If the system worked, the sleaze wouldn't get on the air," said Kathleen Hall Jamieson, a political ad expert at the University of Pennsylvania's Annenberg School for Communication.

Lawsuits take time and money, neither of which the campaigns have to spare. And, proving defamation of character, slander or libel is extremely difficult when the person attacked is a public figure.

So, campaigns often settle on urging stations to reject an ad — while publicly objecting to the content.

"There are incidents where candidates object every cycle and stations do pull ads," said Trevor Potter, a former member of the Federal Election Commission (search). "But there aren't a huge number of them pulled."

Still, stations require political parties and interest groups to back up statements in ads. The opposing side often sends its own documents rejecting the charges and reminding TV stations of their overriding duty as broadcast license holders "to protect the public from false, misleading or deceptive advertising."

Often, documentation from the ad sponsors, whether accurate or not, is all a station needs to put an ad on the air. Sometimes, the station has its lawyers review the material before going forward.

Anticipating a challenge to its first ad, Swift Boat Veterans for Truth sent 28 stations in Ohio, Wisconsin and West Virginia a 12-page background document and 64 pages of material supporting the ad's claims, including affidavits.

Lawyers for Kerry and the Democratic National Committee quickly faxed an objection letter to the stations, pointing out that questions were being raised publicly about claims made by veterans.

The group says two stations didn't run the ad. A couple of others hesitated.

Jeff Armstrong, a station manager for Wisconsin's WLAX and WEUX said the publicity about potential problems with the ad prompted them to initially refuse the commercial. But, he said, they "had a change of heart" after other stations aired it.

Another station, WEAU, an NBC affiliate, waited a day for its lawyers to sign off on the ad, said Steve Lavin, the station's general sales manager.

Sometimes, one side can get stations to force the other side to change an ad.

In January, the Republican National Committee challenged an ad by an arm of MoveOn.org. It said: "Bush sided with the drug companies who had given him huge contributions." Attorney Charles Spies said in a letter to stations in Ohio, Florida and other states that the ad "falsely and maliciously" accused Bush of "committing a federal crime."

"No drug company has ever given a contribution" to Bush because "corporate contributions to federal political campaigns have been legally prohibited for close to 100 years now," Spies wrote.

The RNC says MoveOn had to change the spot before several stations would air it.