The airline, which is the official carrier of the Republican National Convention, asked that the billboard be removed because it is "scary" and "anti-McCain," referring to Arizona Sen. John McCain, the presumptive presidential nominee.

The Union of Concerned Scientists said the billboard is one of two place by the group at the Minneapolis and Denver airports to coincide with the Republican and Democratic presidential nominating conventions.

The group said the ads are aimed at urging the presidential candidates to address the threat of nuclear weapons.

The Minneapolis ad features an image of the city's downtown with crosshairs superimposed on it.

"When only one nuclear bomb could destroy a city like Minneapolis," the headline says, "we don't need 6,000." The subhead says: "Senator McCain: It's time to get serious about reducing the nuclear threat."