WASHINGTON — A burn-it-down candidate is topping a splintered field of more moderate contenders and setting the party’s establishment wing on edge. It’s how Donald Trump began his unlikely march to the Republican nomination in 2016. And four years later, it’s how Sen. Bernie Sanders has cemented himself as the front-runner for the Democratic nomination. The Vermont senator won his second straight contest on Saturday with a convincing victory in Nevada, the first racially diverse state on the primary calendar, after winning the New Hampshire primary the week before.