The president ripped into the former vice president while addressing reporters before leaving for a renewable energy event in the afternoon and later a GOP fundraiser in Iowa, where Biden was campaigning.
Asked which Democratic candidate he'd most like to run against, Trump answered, "I'd rather run against Biden. I think he's the weakest mentally. He's the weakest up here," he said, gesturing to his head. "The others have much more energy."
But on Tuesday night, Trump's remarks at the Iowa GOP fundraiser, just hours after he ripped Biden, offered a stark contrast. Trump didn't even mention the former vice president during his nearly hour-long remarks, nor did he directly go after any 2020 Democratic rivals.
Biden, however, trekked to eastern Iowa, on the heels of a weekend visit by most of the Democratic field, which Biden skipped. There, the former vice president held three community stops, in Ottumwa, Mt. Pleasant and Davenport.
Polls showed a tight race in a hypothetical match-up between Trump and Biden, which makes Iowa an important swing state.
Biden takes on Trump
Biden's swing through eastern Iowa on Tuesday and Wednesday marks his second trip to the crucial state since announcing his presidential run in April, making his pitch to voters during three events.
One thing is abundantly clear about Biden's message this time around: He's taking on Trump, head on.
"It wasn't planned this way, but ... President Trump is in Iowa today," Biden said during his stop in Ottumwa. "I hope his presence here will be a clarifying event."
Biden's remarks marked some of his most pointed attacks of the president, mentioning his name dozens of times and hitting Trump for his tariff policy and the impact it has on Iowa farmers and manufacturers.
"Trump doesn't get the basics. He thinks his tariffs are being paid by China. Any beginning econ student at Iowa or Iowa State could tell you that the American people are paying his tariffs," Biden said in Davenport.
Biden also hit Trump for his denial of climate change.
"[S]till, Trump denies climate change. What did he tell Piers Morgan in that interview? 'Well the weather goes both ways,'" the former vice president said. "It reminds me of when he tweeted in the winter that since it was cold outside there was no climate warming. Or how about when he said the way to deal with California's fires was to rake the leaves? It would be funny if it wasn't so serious."
"I believe," Biden continued, "Trump is an existential threat to America" and a "genuine threat to our core values."
"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," Biden said Tuesday night.
Biden has attempted to take on Trump from the start of his candidacy, calling out the current administration in his announcement video and at his campaign's kickoff rally in Philadelphia, calling the president the "divider-in-chief."
But while Biden has been eager to take on the president in his campaign, he didn't take the bait on Trump's attacks Tuesday morning.
When asked by reporters about the president's comments about Biden's mental fitness, the former vice president brushed it off.
"I'm not going to stoop down to where he is," Biden said after his afternoon event in Mount Pleasant.
Biden has also largely stayed away from taking the bait from his 2020 rivals. He didn't attend the Iowa Hall of Fame dinner Sunday night alongside 19 other 2020 candidates, staying instead in Washington to celebrate his granddaughter's high school graduation with former President Barack Obama.
"[M]y granddaughter was graduating from high school. And her best friend is Sasha Obama," Biden explained. "So Barack and I and Jill and, and the whole family, we got together afterwards to have a little light lunch and dinner for the families that all these girls grew up together with."
While in Iowa last weekend, Biden's opponents mostly refrained from calling the former vice president out by name, instead making veiled swipes at him for his "middle ground" policies and calls for a "return to normal."
Biden addressed those comments in Iowa: "[I]t's clear that Trump is shredding what we believe in most. I believe we have to restore those basic values. I gather some people think that's a return to the past. I don't see it that way. I see it as embracing the enduring values that have made America, America. I don't think that's taking us into the past. For me, it's the only way America is going to have a future."
A new CNN/Des Moines Register poll showed Biden still leading the crowded Democratic field in Iowa, with 24% of likely Democratic caucus voters naming him their top choice.
But in the new poll, Biden failed to add to his lead from a statewide poll in March before he officially entered the 2020 race.
The last time Biden ran for president, in 2008, Iowa provided the final blow to the Delaware senator's campaign. Biden dropped out of the race one day after the Iowa caucus after failing to receive 1% of the vote.
Biden would re-enter the race a few months later, joining former President Barack Obama -- who won the Iowa caucus that year -- as his running mate.
Trump to talk farm policy
In Council Bluffs, Trump was expected to tout his plan to allow year-round sales of gasoline mixed with 15% ethanol, as well as his administration's farm policies. Later that day, he will attend a private fundraiser hosted by the Iowa GOP.
The state's Republican party, which is expecting about 800 people to attend, scrambled to prepare with seven days notice and advertised the dinner as "intimate."
"I know Joe Biden is coming at the same time, but you know what? He's going to be drowned out," Kaufman said. "Poor Uncle Joe, I hope he finds a dinner somewhere where he can talk to three or four people."
Iowa, known for its first-in-the-nation caucus, will still hold a GOP caucus in 2020, giving Republican voters an opportunity to vote against the president.
"I don't really expect [the president] is going to have any competition in Iowa, but we still have to have our caucuses," Kaufman said.
Kaufman said the caucus is a way for the Republican state party to build its infrastructure, especially for a Senate race and four competitive congressional seats.
"People have asked me, 'If he doesn't have any competition, why have the caucus? And I say, 'Well, 2024.' We have [to be] ready in 2024, when there will be competition, and we can't take a cycle off," he said.
But the state GOP did not hold a caucus for the last three Republican incumbents.
For Republicans, the winner of the Iowa caucuses has only clinched the presidency, since 1980, just once: George W. Bush in 2000. Trump placed second, behind Sen. Ted Cruz, R-Texas, in 2016.
It's set to be busy week for the Republican Party, with not only a stop from the president but also former U.S. Ambassador to the United Nations Nikki Haley, who will also be in town for a fundraiser for Sen. Joni Ernst, R-Iowa.
Ernst has cautioned against some of the president's decisions, most recently his strategy of threatening tariffs on Mexico. Still, Kaufman said the party is strong and united.
"She is able to voice her concerns. She is able to disagree with this president, and he listens and he still respects her. No, we're not a monolithic machine here where we all say the same thing ... we actually can disagree and can debate," he said.
A reversal and a rejoice
The former vice president will have to get past 22 other opponents before facing off with the current president, but Biden's recent shift to the left on an anti-abortion measure has raised questions on Biden's primary strategy, and has the Trump campaign ready to pounce.
In recent days, the Biden campaign has been on the defensive after the former vice president said he no longer supported the Hyde Amendment, which bans the use of federal funds for abortions except in cases of rape, incest or where the mother's life is at risk. The switch came just one day after his campaign said he did support the measure.
Biden's initial support drew a wave of criticism from his 2020 opponents, because it largely affects patients who are on Medicaid, meaning low-income patients have to pay for an abortion out-of-pocket.
Biden said his shift in support was a decision he made on his own, after laying out his health care policy with advisers, and realizing there was no way to guarantee low-income women access to the care they need without the use of federal funds.
"I still believe if, in fact, there was a means by which poor women would be able to access their constitutional right and it was able to be paid for without taxpayers' money, it should be done," Biden told reporters Tuesday. "It's a constitutional right, and it can't be denied. When there was an alternative, it made sense. But now there's virtually no alternative."
The reversal marked a leftward shift for Biden, who has been viewed as a moderate candidate and has campaigned on his ability to work across the aisle. Biden faced some vocal criticism on his shift while in Iowa -- all three events had a protester interrupt Biden's remarks to ask about abortion. The shift also raised questions about Biden's ability to withstand attacks from progressive challengers and grassroots movements.
And following Biden's reversal, the Trump campaign, bolstered by pro-Trump groups, also piled on the criticisms, dinging the Democrat for what appeared to be a critical shift to the left.
"He's just not very good at this. Joe Biden is an existential threat to Joe Biden," Tim Murtaugh, Trump's campaign communications director, told ABC News in a statement.
America First Action Super PAC and America Rising PAC, both major pro-Trump groups, flooded Twitter with messages calling out Biden's reversal just minutes after news broke regarding his new comments.
A senior adviser for the Biden campaign told ABC News that the political pressure from other 2020 candidates did not have an impact on Biden's decision, saying, "there were spirited discussions about the Hyde Amendment over the last couple of weeks" but that ultimately the decision came to a head last week and Biden decided to address it in Atlanta.
The former vice president's reversal on the Hyde Amendment is exactly the kind of shift to the left that the Trump campaign and these pro-Trump groups have been waiting for, according to conversations with those groups. Many who are working to re-elect the president have been betting that Biden would need to tilt further and further toward progressive policies just to escape the primary, and the former vice president's reversal on the Hyde amendment is the best example of that so far.