Gov. Janet Mills’ exit from the Maine Senate race handed Senate Minority Leader Charles E. Schumer another public defeat, sharpening doubts about his political instincts and his hold on Senate Democratic leadership. Her departure also underscored a deeper problem for Mr. Schumer: the candidate he tried to sideline has become the party’s strongest contender. Graham Platner — a 42‑year‑old former Marine and oyster farmer who has electrified progressives — has consistently polled as Democrats’