It's primary day in Maine, North Dakota, Nevada and South Carolina, where voters are casting ballots in a series of key Senate, House and governor's races. For Democrats, it's all about the Pine Tree State, where it isn't so much about picking a nominee, but testing the one that they're pretty sure they already have. Maine voters will deliver the first real evidence of whether Graham Platner's latest controversies have diminished his support, and nothing less than the future of the Senate may be riding on his candidacy. CNN's Arlette Saenz kicks off our coverage at a polling station in Portland.  

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