Federal prosecutors are trying a novel legal tactic to strike at the heart of what they consider a notorious outlaw motorcycle gang: using trademark law to take away its treasured logo.

In a pending racketeering trial in Los Angeles, the Justice Department will attempt to legally seize the black-and-white emblem worn on leather jackets by members of the Mongols Motorcycle Club, which depicts a muscled, ponytailed warrior with a Fu Manchu mustache speeding on a chopper. The trial was to begin in early June but was delayed last...