Fox News reporter startled by Trump's Biden-bashing rant: I asked him a question and he just went off
Fox News White House correspondent John Roberts. (Screenshot)

Fox News White House correspondent John Roberts on Tuesday said President Donald Trump "just went off" after he asked a question about his potential 2020 rival, Democratic candidate Joe Biden.

“You’d think it’s already down to the general election in a two-person race, because Joe Biden in a speech later on today — which we’ve got an advanced copy of the comments of what he will say in the speech — he really takes on President Trump as if they are running head-to-head in the general election," he said.

“And today on the South Lawn, as the president was leaving for Iowa himself where he will give a speech on renewable energies and then do a campaign style event tonight, I asked the question, Joe Biden is going to say in the speech that you and your policies represent an existential threat to this nation," Roberts continued.

"And the president just went off on Biden, calling him basically 1% Joe, not mentally very strong -- wait until you hear the comments. This is a back and forth that you would not expect some 17 months out before the election."

In his comments, Trump said that Biden was a "loser" who was "slower than he used to be."

"I would rather run against Biden than anybody. I think he’s the weakest mentally. I like running against people who are weak mentally," the president said.