https://cdnph.upi.com/ph/st/th/6251781013689/2026/upi/32f236de8ec682b11e763d84f0c5b7ad/v1.5/Voters-in-Maine-South-Carolina-Nevada-North-Dakota-head-to-polls.jpg" /> Five-Term Republican Sen. Susan Collins will learn who she will face in November as voters head to the polls in Maine's primary election on Tuesday.