Sen. Bernie Sanders of Vermont has a commanding and stable lead over Hillary Clinton in New Hampshire ahead of the Tuesday’s Democratic primary, enjoying a 20-point edge that budged little after Iowa’s closely contested caucuses, a new Wall Street Journal/NBC News/Marist poll finds.

Powered by lopsided support among millennials, Mr. Sanders is the first choice of 58% of likely Democratic primary voters in New Hampshire, compared with 38% who favored Mrs. Clinton, the former secretary of state.

...