The spectacular implosion of Graham Platner’s high-profile campaign for Maine’s U.S. Senate seat over a sex assault allegation has created a huge headache for Democrats in race that is seen as crucial to their long-shot fight to retake the Senate in the fall midterm elections.Feuding Maine Democrats are squabbling over who will replace the scandal-tarred Platner and resume the fight to unseat five-term Republican Sen. Susan Collins, a must-win contest in their uphill struggle to flip at least four GOP-held seats and grab back control from President Trump’s allies. At least a half-dozen candidates have thrown their hats in the...