The war of words between Donald Trump and ex-Mexican President Vicente Fox went nuclear yesterday when Fox suggested that Trump reminds him of Hitler.

Previously, Fox had responded to a question about Trump's proposed wall that he says Mexico will pay for by dismissing the idea.  "I'm not going to pay for that f****** wall," Fox said.

Trump shot back: “FMR PRES of Mexico, Vicente Fox horribly used the F word when discussing the wall. He must apologize! If I did that there would be a uproar!"

For the record, Trump dropped five F-bombs in a little more than a minute at a rally last month in New Hampshire.

"He has offended Mexico, Mexicans, (and) immigrants. He has offended the Pope. He has offended the Chinese. He's offended everybody."

Fox's comments come one day after he delivered a scathing response on Trump's plan to make Mexico pay for a wall between the Mexico-U.S. border.

"I'm not going to pay for that f***ing wall," Fox said in an interview with Fusion's Jorge Ramos.

Fox told Cooper he won't apologize for that remark.

Fox isn't the first to suggest Trump's rhetoric is similar to that of the German dictator.

Last month, Anne Frank's stepsister accused Trump of "acting like another Hitler." And in December, former New Jersey Gov. Christine Todd Whitman invoked Hitler when discussing Trump's plan to temporarily ban Muslims from entering the U.S.

"If you go and look at your history and you read your history in the lead-up to the Second World War, this is the kind of rhetoric that allowed Hitler to move forward," Whitman told CNN's Alisyn Camerota on "New Day."

Invoking the specter of Hitler used to go against a major unwritten rule in politics.  However, Democrats have been doing it for years when referring to conservatives, so the epithet has lost much of its bite.

America is not Weimer Germany, so some obvious parallels between Trump and Hitler go only so far.  Still, it's worrisome that Trump demonstrates a towering ignorance of the Constitution and due process – reason enough to keep a watchful eye on him on the off-chance he actually gets elected.