https://cdnph.upi.com/ph/st/th/5181781103519/2026/upi/40632486d7151db4f39253e30d0ec36d/v1.5/Platner-wins-Maine-primary-Mace-loses-SC-governors-race.jpg" /> Maine Democrat Graham Platner secured his party's nomination to challenge Sen. Susan Collins in November in one of Tuesday's key midterm primaries.