“Former Maine Senate President Troy Jackson filed with the Federal Election Commission on Tuesday to form a U.S. Senate exploratory committee, becoming the first Democrat to formally campaign for the spot Graham Platner appears likely to vacate,” Bangor Daily News reports.Punchbowl News: “But some progressive groups are already raising questions about Jackson’s past. During the governor’s race, Jackson was hit by ads accusing him of opposing abortion rights. And Jackson’s past positions have already been called into question.”