(CNN) Sen. Kamala Harris calls President Donald Trump a "fraud." Former Vice President Joe Biden says Trump is "literally an existential threat to America." South Bend, Indiana, Mayor Pete Buttigieg wants to "change the channel" on the President.

2020 Democratic presidential candidates are attempting to demonstrate their toughness and ability to draw sharp contrasts with Trump to the party's primary voters.

Polls show why it's a crucial skill: In choosing their nominee, Democratic voters are most concerned about the ability to defeat Trump. And a CNN poll released last week showed 54% of Americans believe Trump will win re-election in 2020.

As the pace of candidate forums accelerates and debates start, the Democratic candidates are road-testing their lines of attack.

The latest up was Biden , who campaigned in Eastern Iowa on Tuesday as Trump visited Western Iowa.

It was split-screen television all day: Biden's campaign released excerpts of his escalated attack on Trump, forecasting that he would later call the President an "existential threat to America" and claim that he "doesn't get the basics" on trade.

Trump, on his way to Iowa, couldn't resist responding.

He called Biden "the weakest mentally" in the Democratic field and a "loser" who is relevant only "because Obama took him off the trash heap."

"His whole campaign is to hit Trump," the President said of Biden. "When a man has to mention my name 76 times in a speech, that means he's in trouble."

The former vice president later mocked Trump for watching Biden's first of three campaign events of the day on Air Force One en route to Iowa.

"I guess he is really fascinated with me," Biden said.

And he poked at Trump for telling reporters he had a document with all the details of a new deal with Mexico but that he couldn't share them yet.

"Apparently, he has a secret important document with Mexico, and I'm sure that there are a heck of a lot of Iowans being crushed by his tariffs that would like to see that secret document," Biden said. "I'm anxious to see it."

In Davenport on Tuesday night, Biden egged on Trump again, mocking his claim at the 2016 Republican National Convention that "only I can fix" America's broken political system.

"Fix yourself first, Donald Trump," Biden said.

Biden's comments came two days after 19 of the 23 candidates in the Democratic field had tested their arguments against Trump at the Iowa Democratic Party's Hall of Fame dinner.

Democratic contenders are seeking ways to play up their own backgrounds and advance their own agendas. But they appear to be attuned to the reality that attacks on Trump -- particularly when they bait him, Vice President Mike Pence and other senior GOP officials into responding -- are the best way to create moments that drive news coverage and go viral on social media.

Their comments in recent days and weeks offered windows into their different approaches to taking on Trump -- as well as the divide among more progressive candidates who see the President as the result of a long-corrupted system that needs a bigger overhaul.

Harris is a former California attorney general whose skills as a prosecutor have been on display when she has grilled Trump administration officials and nominees in Senate committee hearings. She leaned into that background in a riff at the Hall of Fame dinner Sunday, casting Trump as a "fraud" who had broken his promises to voters and saying she would "prosecute the case."

"He promised health care and then he tried to rip health care away from millions of people. What's that called? Health care fraud," she said. "He said he was for working people, then he passed a tax bill benefiting the top 1% and the biggest corporations in this country. That's tax fraud. He believes the President of Russia and a North Korean dictator over the word of the American intelligence community. Securities fraud."

"And then he claims to be the best President we've seen in a generation," Harris said. "Well, I say let's call Barack Obama, because that's identity fraud."

Sen. Amy Klobuchar of Minnesota is attempting to build her image as a tough Midwesterner. She recounted announcing her presidential campaign in the middle of a snowstorm, and Trump mocking her for "talking proudly of fighting global warming while standing in a virtual blizzard of snow" and calling her "Snowman(woman)" on Twitter.

"So I wrote back, 'Hey, Donald Trump, the science is on our side, and I'd like to see how your hair would fare in a blizzard,'" she said.

Buttigieg, the mayor who saw his stock rise among Democratic primary voters after assailing Pence, a fellow Indiana politician, over the vice president's stance on LGBT issues, vowed to "change the channel to something completely different."

He repeated that line on Tuesday, but told reporters he didn't want to get into a name-calling match with Trump.

"It may come down to name calling on his side, but I'm not going to play that game. Look, you can't go on his show and expect to win. So the only response to his show is to change the channel, and that's what we're trying to do in this campaign," Buttigieg said.

Sen. Bernie Sanders of Vermont said Trump is "a liar, a racist, a sexist, a homophobe, a xenophobe, a religious bigot and someone who believes that he is above the law."

But the message Sanders is delivering on the trail, which his campaign has hammered away at in emails to supporters, is that voting Trump out of office is only part of the fight to come.

"This election needs to be about so much more than beating Donald Trump. It is a mistake to think that this election is simply about beating one man -- an aberration of a president -- and that everything will simply return to 'normal,' " Sanders campaign manager Faiz Shakir wrote in early May. "The reality is that 'normal' in our country before there was a President Trump still meant an immoral lack of healthcare, unlivable low wages, rampant corporate greed, a racist criminal justice system, and a corrupt political system."

That word -- "aberration" -- had been used by Biden to describe Trump often, including in the video announcing his entry into the race, when he said, "I believe history will look back on four years of this President and all he embraces as an aberrant moment in time."

Both Sanders and Sen. Elizabeth Warren of Massachusetts, who has jumped up in the polls while pitching "big, structural change" to American life -- well beyond the immediate controversies and scandals swamping the administration -- have been adamant that Trump is the product of a deeply corrupt system that needs to be rooted out and re-imagined.

"The man in the White House is not the cause of what is broken, he is just the latest and most extreme symptom of what's gone wrong in America," Warren said of Trump during her campaign kickoff in February. "A product of a rigged system that props up the rich and powerful and kicks dirt on everyone else. So once he's gone, we can't pretend that none of this ever happened."

During her remarks at the California Democratic Party convention earlier this month, Warren put it simply. "I'm here because in 2020, we have a job to do: Beat Donald Trump," she said. "But beating Trump is not enough."