SAN FRANCISCO—A group of Amazon.com Inc. workers in New York City’s Staten Island have withdrawn a petition to hold a union election, marking a setback in a growing push by organized labor to bring the online retailer’s staff into the fold. The group, known as the Amazon Labor Union, which is not affiliated with a major U.S. union, filed its petition last month, joining union campaigns underway at Amazon facilities in Alabama and Canada. In order to hold an election supervised by the U.S. National Labor Relations Board, unions must show that they have gathered signatures of support from at least 30 percent of workers who are eligible to vote. Chris Smalls, president of the group and a former employee at the Staten Island warehouse, said Friday the group opted to withdraw its petition after the NLRB informed them they had not collected enough signatures. “We will resubmit as soon …