Voters across Maine, Nevada, South Carolina and North Dakota will cast their ballots Tuesday in another day of primary elections in America, but much of the political world will be focused on Maine’s high-stakes U.S. Senate contest.

The results aren’t in question. Neither Republican incumbent Sen. Susan Collins nor Democratic challenger Graham Platner faces serious opposition for their party’s nomination. And yet Tuesday marks an especially significant moment for Platner, the embattled veteran and oyster farmer, who’s fighting to rebuild his credibility in a campaign rocked by controversy.