With Graham Platner’s campaign teetering from an allegation of sexual assault, there’s no shortage of people arguing that the 41-year-old Maine oysterman and progressive political neophyte was always too risky for a key Senate race. However, it’s likely more candidates will follow his path in the future.That’s because Platner’s starburst candidacy followed an increasingly familiar arc that’s shown no signs of abating in American politics — the outsider who defies the warnings of party veterans and captures the hearts of primary voters, even as liabilities pile up and complicate the odds of winning the general election.With candidates having so many…