Donald Trump and Ben Carson continue to broaden their appeal among Republican primary voters and have widened their lead over former Florida Gov. Jeb Bush and many other more-experienced candidates, a new Wall Street Journal/NBC News poll finds.

Mr. Bush, once considered the GOP’s likely nominee, is also lagging behind his onetime protege, Florida Sen. Marco Rubio, who is emerging as a leading contender to rally the party’s establishment wing against the rise of insurgent outsiders such as Messrs. Trump and Carson.

The...