Joe Biden, busy in the last week flip-flopping and plunging in the polls, debuted his hoped-for campaign against President Trump on Tuesday during a trot through Iowa.

And so did Trump, who has been hponig to face off with Biden in 2020.

“As Democrats, we often say Trump poses a threat to this nation. But what do we mean?” Biden said, according to a transcript released before his speech.

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“Sometimes I think it’s said so often we don’t realize just how serious a threat he is — and why. I believe Trump is an existential threat to America. I’ve said many times that we can overcome four years of Trump — but if we give him eight years in the White House he will forever and fundamentally alter the character of this nation.

“So — what is the threat? I believe Trump is an existential threat to America,” Biden said, calling Trump by name nearly 80 times in his speech.

“First, to our core values. What we stand for. What we believe,” the former vice president said, bringing up the racial clash last year in Charlottesville.

“Second, to our standing in the world. Trump just attended the 75th Anniversary of D-Day — one of the most important symbols in the world for the importance of America’s alliances,” Biden said before noting that Trump slapped singer Better Midler for sending out a “fake news” post on Twitter. “And third, our democracy. Our democracy is at risk. I never thought I would say those words. But it’s true. Everywhere you turn, Trump is tearing down the guardrails of democracy. It’s the abuse of power. And if there is one thing I can’t stand — it’s the abuse of power. Whether it’s a man raising his hand to strike a woman or a child, or an employer treating his workers like dirt, or an American president who has no respect for the rule of law — I can’t stand for it,” Biden said. For his part, Trump hit back fast — and hard.

‘When a man has to mention my name 76 times in a speech, he’s in trouble,” Trump said on the White House lawn before leaving hfor his own trip to Iowa. “Now, I have to tell you, he’s a different guy. He looks different than he used to, he acts different then he used to. He’s even slower than he used to be. So I don’t know,’ the U.S. president said. ‘But when he mentions my name that many times I guess I should be complimented.”

“I like running against people that are weak mentally,” he said. “I think Joe is the weakest up here. Joe Biden thought that China was not a competitor of ours. Joe Biden is a dummy,” he said. “China is a competitor.”

Then Trump let it rip.

“Well, I heard Biden, who’s a loser — look, Joe never got more than 1 percent except Obama took him off the trash heap and now it looks like he’s failing,” the president said. “It looks like his friends from the left are going to overtake him pretty soon, but I heard, you know, his whole campaign is to hit Trump,” he added, referring to Biden’s plunging poll numbers and the rise of Sens. Bernie Sanders and Elizabeth Warren.