Former Maryland Gov. Martin O'Malley (D) on Monday declared that Donald Trump is a "fascist demagogue."

Following Trump's suggestion on Monday afternoon that the US should institute a "total and complete shutdown of Muslims entering the United States" for the time being, the presidential candidate tweeted that Trump had "remove[d] all doubt."

O'Malley, who has lagged behind front-runner Hillary Clinton and Sen. Bernie Sanders (I-Vermont) in primary polls, has repeatedly criticized Trump for his inflammatory remarks about immigration.

"The fact of the matter is, and let's say it in our debate — because you'll never hear this from that immigrant-bashing carnival-barker Donald Trump — the truth of the matter is that net-immigration from Mexico last year was zero," O'Malley said during the Democratic presidential debate last month.

Trump — who has dismissed O'Malley as a "disgusting, little, weak, pathetic baby" — immediately responded in a tweet.