PORTLAND, Maine (AP) — Graham Platner has submitted his paperwork to formally withdraw from Maine’s U.S. Senate race, officially ending an upstart yet troubled campaign whose dissolution threatens Democrats’ pursuit of chamber control. The Maine Democratic Party announced later Friday it will hold a nominating convention on July 25 to choose his replacement on the November ballot. Platner’s paperwork was received Friday by the Maine secretary of state’s office. Maine is considered a key state for control of the narrowly divided Senate, and Democrats were desperate for a candidate capable of defeating Republican Sen. Susan Collins.