Divisive rhetoric on issues of race is nothing new in American politics, but one presidential historian says it's never been quite as inflammatory as what is being seen now from Donald Trump's Republican presidential campaign.

Modern politics has never seen such a brazen and polemical presidential candidate, said Julian Zelizer, professor of history and public affairs at Princeton University and author of several presidential biographies.

"I've thought a lot about this because people are always asking me for what the historical precedent is, and what we've seen is there have been many politicians who have played on some of these divisions in society, but they've also felt the need when they were the candidate to contain their rhetoric," Zelizer said.