DETROIT — Republican James Craig, the former Detroit police chief, is suspending his campaign for Michigan’s open U.S. Senate seat and weighing running for mayor of Detroit, he told The Detroit News in an interview Tuesday. Craig said while he viewed himself as the leader for the GOP nomination for U.S. Senate, he had made a “business decision” because of concerns about the cost of obtaining and verifying the required 15,000 valid petition signatures he needed by April 23 to make the primary ballot. “The reality is fundraising is a challenge right now,” Craig told The News. Craig’s campaign re…