BOULDER, Colo. -- This city has always taken pride in its liberal-to-the-point-of-loony reputation. But this Halloween, one of its wackiest traditions is under siege: the Naked Pumpkin Run.

The event is exactly what its name implies. Scores of men and women pour into downtown streets for a late-night jog, wearing not a stitch between the jack-o'-lanterns on their heads and the sneakers on their feet.

For...