An advocacy group seeking to curtail the proliferation of nuclear weapons is crying foul after an ad aimed at presumptive GOP nominee John McCain was ordered removed from Minneapolis-St. Paul International Airport.

The group, the Union of Concerned Scientists, took out ads in the Denver and Twin Cities airports aimed at both presidential candidates. The MSP ad shows an overhead photograph of downtown Minneapolis and says, "When only one nuclear bomb could destroy a city like Minneapolis ... We don't need 6,000."

But Northwest Airlines officials have ordered the ad removed.

"This is a private airline acting as a political censor," said Elliott Negin, media director for the Union of Concerned Scientists.

The ad urges McCain to get serious about the nuclear threat. The Denver ad, aimed at presumptive Democratic nominee Barack Obama, is similar.

Thousands of delegates, party dignitaries, media members and others are expected to pass through the airport on their way to the convention, to be held Sept. 1-4 at St. Paul's Xcel Energy Center.

The airport ad is part of a broader advertising effort aimed at both conventions, Negin said, one that will include radio spots and smaller ads in bars and restaurants,

Negin said the UCS also bought ads on the sides of cabs in Boston and New York City during the 2004 conventions and received no complaints.

But Northwest - the official airline of the Republican National Convention - asked its advertising vendor, ClearChannel Outdoor, to remove the ad. It had been up since Aug. 13 and was supposed to stay up for one month.

In e-mailed correspondence between Northwest and ClearChannel released by the UCS, Northwest regional director Kathleen Nelson asks for the ad to be removed.

"I just took a look and I can see how this would be offensive/scary to some (the concept of our city in the crosshairs of a nuclear bomb) and the strong anti-McCain message. Can we remove it?" she wrote.

In an e-mail to the Pioneer Press on Monday, Tammy Lee, Northwest's vice president of corporate communications, said, "Out of an abundance of respect for both parties, we will not allow attack ads of either persuasion to be prominently displayed in our concourses. Our customers and employees complained and we responded. We will not be a party to petty political attacks on either side."

Negin disputes that the ad is anti-McCain.

"We'll agree that nukes are scary, and that's why we think more people should be paying attention to (the issue)," he said. "To say that the ad is anti-McCain is ludicrous."

Negin said the UCS does not endorse political candidates.

Northwest controls advertising in Concourse G, where Northwest flights originate and depart.

Peggie Hardie, general manager for Clear Channel Airports in Minneapolis, told the Associated Press that Clear Channel has asked the UCS to modify the ad and will accept a revised version if its concerns are met.

Negin said the UCS would be willing to discuss the ad with Hardie. But he noted that Clear Channel had already accepted and posted the ad, and said the company should let it stay up.

Such an ad wouldn't have been allowed in other parts of the airport, where advertising is controlled by the Metropolitan Airports Commission. Only commercial and other nonpolitical advertisements are allowed, MAC spokesman Patrick Hogan said.