President Trump and former vice president Joe Biden are both in Iowa Tuesday, as Biden continues to criticize the president and the president continues to lob insults at the Democratic frontrunner.

Internal Trump campaign polling shows Mr. Trump trailing the former vice president in key states, despite Mr. Trump's public confidence in his path to victory. As he left the White House Tuesday, Mr. Trump told reporters Biden is "weak mentally," adding, "I like running against people who are weak mentally." Those comments came as Biden in an Iowa speech declared the president an "existential threat" to the nation's future.

"I'd rather run against I think Biden than anybody," Mr. Trump told reporters Tuesday. "I think he's the weakest mentally. And I like running against people that are weak mentally. I think Joe is the weakest up here. The other ones have much more energy. I don't agree with their policies, but I think Joe is a man who is, I call him one percent Joe. Because until Obama came along he didn't do very well. But look, but I don't bring him up."

Get Breaking News Delivered to Your Inbox

But the president does bring him up on his own. During his speech on ethanol and renewable fuels in Iowa Tuesday afternoon the president suggested "sleepy Joe" — one of his nicknames for Biden — held Americans hostage to foreign energy suppliers.

"America must never be held hostage to foreign suppliers of energy as we have under sleepy Joe," Mr. Trump said.

Mr. Trump is in Iowa to speak at the Southwest Iowa Renewable Energy Company, and at a GOP fundraiser, where he's expected to raise hundreds of thousands of dollars. Iowa has been hard hit by the trade war with China. On his way to Iowa, Mr. Trump insisted he's the greatest thing that's ever happened to America's farmers, touting the $16 billion in subsidies he recently gave the farming community to offset his trade war with China. The president did sign an executive order to modernize the regulatory framework for agricultural biotechnology products, meant to speed up the approval process.

But Mr. Trump's beef with Biden doesn't show signs of ebbing.

Last week, Mr. Trump appeared to bash Biden while he was overseas, agreeing with North Korean Leader Kim Jong Un in questioning Biden's intellectual chops.

"I was actually sticking up for Sleepy Joe Biden while on foreign soil," Mr. Trump tweeted last month, disputing claims he sided with a dictator over an American former vice president. "Kim Jong Un called him a 'low IQ idiot,' and many other things, whereas I related the quote of Chairman Kim as a much softer 'low IQ individual.' Who could possibly be upset with that?"

Moments after Mr. Trump returned to American soil, Biden's campaign manager labeled Mr. Trump's remarks "beneath the dignity of the office."

Mr. Trump has yet to publicly kick off his reelection campaign — he's doing that in Florida later this month — but the president filed for reelection the same day he took office.