WASHINGTON — President Donald Trump couldn't wait to arrive in Iowa before attacking Joe Biden. And vice versa.

With the two rivals both campaigning in the state later Tuesday, and a potential 2020 election matchup between the two 70-something white men coming into clearer focus, the bitter back-and-forth began early and kept up all day, a possible sign of the tumult to come.

Biden's campaign fired the first shot, releasing excerpts of a speech he plans to deliver Tuesday night in Davenport in which the former vice president calls Trump "an existential threat" to the nation and appeals to Midwestern farmers and autoworkers by blasting the president's tariffs for "choking" their industries.

Trump fired back as he left the White House around noon for visits to Council Bluffs and Des Moines. In a back and forth with reporters on the South Lawn, he dismissed Biden as "a loser" for getting only 1% in Iowa's Democratic caucuses when he ran for president in 2008, which ended his nascent campaign, and polls showing a tightening race now.

"It looks like he's failing," Trump said. "It looks like his friends from the left are going to overtake him very soon."

Trump added that he would "rather run against, I think, Biden than anybody. I think he's the weakest mentally, and I think Joe is weak mentally. The others have much more energy."

Trump has been obsessed with Biden, partly because his campaign's internal polls show Biden leading the president in critical swing states like Pennsylvania and Michigan, both of which Trump unexpectedly won in 2016 and probably needs again if he is to win reelection.

Biden, seeking to position himself above the squabbling multitude of his party's presidential race, has focused largely on drawing contrasts with Trump. And by releasing a speech text in advance of his Iowa tour, Biden's campaign provoked the president and garnered more attention than it might have gotten otherwise as cable stations broadcast his first comments live in the middle of the afternoon.

In the near term, Iowa's first-in-the-nation contest on Feb. 3 is far more important for Biden than for Trump, who is virtually assured of capturing his party's nomination absent an unlikely political earthquake. Another major loss for Biden in the state could hobble his campaign again.

Despite his front-runner status, Biden has yet to win over Iowa's Democratic activists — Tuesday's three stops come in his first appearance in the state, and he began his remarks with an apology for his decision to skip a forum Sunday in Cedar Rapids that drew 19 candidates.

A weekend Des Moines Register poll showed Biden atop the Democratic field with 24%, a significant share considering there are two dozen candidates in the race but notably smaller than his current lead in most national polls.

Three candidates are tightly packed in a second tier behind Biden in the newspaper's poll: Sen. Bernie Sanders with 16%, Sen. Elizabeth Warren with 15% and Mayor Pete Buttigieg with 14%.

But Trump's fixation on the former vice president is helping Biden execute his primary strategy of casting himself as an inevitable and formidable general election challenger to the incumbent.

According to Biden's prepared text for Davenport, he will focus on the impact on American farmers and blue-collar workers of Trump's escalating trade tariffs on China and other countries — and he'll mention the potential political impact as well on the White House.

"Trump doesn't get the basics. He thinks his tariffs are being paid by China," Biden planned to say about Trump. "Any beginning econ student at Iowa or Iowa State could tell you that the American people are paying his tariffs. The cashiers at Target see what's going on — they know more about economics than Trump."

Trump, clearly irked, called Biden "a dummy" who doesn't take China seriously enough as a threat. "China ate our country alive" during the Obama administration, when Biden was vice president, Trump charged.

Biden's first event of the day was in Ottumwa, in eastern Iowa, where his words were tough but his manner was restrained as he read from prepared remarks.

"President Trump is in Iowa today," he said. "I hope his presence here will be a clarifying event, because Iowa farmers have been crushed by his tariff war on China. He thinks he's being tough. It's easy to be tough when someone else feels the pain."

He derided Trump for announcing and then backing away from slapping tariffs on Mexico, attributing the president's change of heart to his fearing the political and economic blowback.

"He realized that he was likely to lose Michigan, Ohio and Iowa," he said. "He's seen the Lord. He had, as we say in Delaware, an altar call."

Brad Parscale, Trump's campaign manager, on Twitter mocked Biden's delivery as he flew west. "The entire row on my flight today is watching Biden speak. They are all now asleep."