Northwest Airlines is grounding a politically charged ad at the Minneapolis-St. Paul International Airport.

The ad urges presumptive GOP presidential candidate John McCain to "get serious" about reducing nuclear weapons.

The timing of the ad, placed by the Washington, D.C.-based Union of Concerned Scientists, is tied to the Republican National Convention, which begins in St. Paul in less than two weeks. The group placed a similar ad for Barack Obama at the airport in Denver, the site of next week's Democratic National Convention.

Northwest officials said the ad, which had been up in the G concourse since Friday, prompted complaints. The $9,400 ad, as well as an unrelated anti-Obama ad, are expected to be removed today.

"We will not allow attack ads of either persuasion to be prominently displayed in our concourses," NWA spokeswoman Tammy Lee said.

The ad shows an aerial image of Minneapolis with a target focused on downtown, with the message: "When only one nuclear bomb could destroy a city like Minneapolis ... we don't need 6,000. Senator McCain: It's time to get serious about reducing the nuclear threat."

Elliott Negin, a Union of Concerned Scientists spokesman, said Northwest is censoring political speech. "To say that this is anti-McCain is ludicrous," he said.

The nonprofit's $5,900 ad addressing Obama remains up in Denver's airport, Negin said.

"We're calling on both candidates and their respective parties to address this," he said. "There's nothing offensive about the ads. They're not scary. It's just words and a photograph."

Terry Collins • 612-673-1790