Joe Biden called out President Trump as an "existential threat" to the U.S. in a campaign appearance in Iowa Tuesday. The bashing by the front-runner for the 2020 Democratic presidential nomination and former vice president came as he and Mr. Trump travel throughout Iowa Tuesday trying to galvanize their supporters.

Biden labeled Mr. Trump an existential threat to decency, America's standing in the world, and democracy.

Biden drew a "sharp contrast" between himself and the president on several key issues, among them tariffs, climate change and restoring American "values."

Throughout his less-than-two-month-old campaign, Biden, 76, has focused his pitch for the Democratic nod on his contention that he is the candidate most likely to beat the president in the general election.

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Biden charged that American farmers "have been crushed" by Mr. Trump's "tariff war with China," which Biden call the product of the president's lack of empathy for rural residents.

Former Vice President Joe Biden speaks during a town hall meeting on Tuesday, June 11, 2019, in Ottumwa, Iowa. Matthew Putney / AP

On his first trip to Iowa in late April, Biden was criticized by some who said he sounded naïve about the potential threats from China when he told an Iowa City crowd in part, "They're not bad folks, folks."

"We should invest in ourselves," Biden said Tuesday about more effective ways to combat China's trade practices. "We should be rallying our friends to take on China's abusive practices around the world."

On climate change, Biden chided the president's previous climate-related comments, saying, "It would be funny if it wasn't so damaging and serious."

Beyond policy and international relations, Biden made an emotional appeal about Mr. Trump's behavior, such as his recent name-calling of Bette Midler on Twitter during his visit to Europe for the D-Day 75th anniversary celebrations. He also referred to Mr. Trump as a threat to democracy.

"Four years of Donald Trump would be an aberration in American history. Eight years will fundamentally change who we are as a nation," Biden said.

He pushed back against criticism that he is too moderate and too willing to collaborate with Republicans, saying that bipartisanship should not be considered "old-fashioned."

"He says, 'Let's Make America Great Again.' Let's make America 'America' again," Biden said. He also pushed back against the tax cuts enacted in 2017, promoting instead his plans to assist Americans with child care and affordable college.

Biden drew a contrast with Mr. Trump in another way during the speech: by acknowledging an anti-abortion protester.

"Let him go, this isn't a Trump rally," Biden said, when audience members began to shout down the protester. He also promised to sit with the man after of the speech.

Grace Segers contributed to this report