Mark Osmack is the kind of candidate that Democratic strategists dream of: a guy with blue-collar roots (lived in a double-wide as a kid), an Army veteran who did two tours in Afghanistan, a St. Louis County native who just so happened to grow up in a district that is suddenly, and decidedly, in play. Oh, and he's also an unabashed progressive — one who can speak persuasively about his beliefs to the guys he grew up with in Fenton.…