25-year-old Ann Arbor man is challenging U.S. Rep. John Dingell for his seat, claiming the long-time congressman isn't progressive enough.Daniel Marcin, who is pursuing a doctorate in economics at the University of Michigan, filed nearly 1,800 petition signatures with the state on Monday to appear on the ballot.Running as a Democrat, he'll face Dingell, D-Dearborn, in the Aug. 7 primary, competing in Southeast Michigan's newly drawn 12th District."I'm definitely to that guy's left," Marcin said of Dingell, attacking the 85-year-old congressman's record on environmental issues, same-sex marriage and tax policy."John Dingell acts like he's Mr. Senior Congressman," Marcin said. "If...