Sen. Cory Booker, D-N.J., said that watching Trump's rally took him back to his parent's time, to the language used by late Alabama governor and presidential candidate George Wallace, who proudly advocated for segregation and fought against the civil rights movement.
Booker, in an interview with The Washington Post Live on Thursday, said the 2020 election is about more than Trump.
"The referendum in this election is not on one guy in one office," he said. "The referendum in this election is a referendum on who we are, and who we are going to be to each other, and if we can get back to seeing each other with a more courageous empathy, we can have a revival of civic grace and create a new American majority."
In many ways, he was summing up a sentiment seemingly shared by the Democratic presidential hopefuls, who squabble over policy differences on the trail but appear united as they confront Trump over his divisive comments.
The undercurrent of the brawl tees up an election fight that is, as Booker said, not only a referendum on Trump's presidency but on the identity of the country.
Booker's comments came after the president amplified his attacks from the weekend at a campaign rally Wednesday night in Greenville, North Carolina, in which he told four freshman members, Reps. Alexandria Ocasio-Cortez, D-N.Y., Ilhan Omar, D-Minn., Rashida Tlaib, D-Mich., and Ayanna Pressley, D-Mass., to "go back" to their home countries, telling the fervent crowd, "I said I have a suggestion for the hate-filled extremists who are constantly trying to tear our country down. They never have anything good to say, that's why I say, hey, if they don't like it let them leave. Leave, let them leave."
Three of the women were born in the U.S., and Omar, who came to the U.S. as a refugee as a child, has been living in the country since she was 12 years old and is a naturalized U.S. citizen. In the 2018 midterms, all four women won a popular vote to claim their seats in Congress.
Trump continued to dig in on his home turf, inflaming tensions as he targeted the cohort of lawmakers, known as the "squad," particularly the Somali-born Omar.
"Omar blamed the United States for the crisis in Venezuela," Trump said, eliciting scattered "send her back" chants from the audience of supporters. "I mean, think of that one. And she looks down with contempt on the hard-working Americans, saying that ignorance is pervasive in many parts of this country."
The cheers from the crowd grew louder as Trump continued: "And obviously and importantly, Omar has a history of launching vicious anti-Semitic screeds."
The chants, which hearken back to calls of "lock her up" during the 2016 election that are still invoked among some Trump supporters, even as a new parade of Democrats are challenging him in 2020, echo the tenor of Trump's Sunday tweets, in which he wrote, "Why don't they go back and help fix the totally broken and crime infested places from which they came."
Amid a steady stream of backlash from Democrats and even a few Republicans on Thursday, the president has since disavowed the chants, telling reporters in the Oval Office he "was not happy with it" and "I disagree with it."
While this most recent campaign rally provided Trump with a platform to energize his base, ahead of a tough reelection fight, it also presented an opportunity for the Democrats, who have tried to cast 2020 as a battle for the "soul" of the country, to fill what they see as a moral void in leadership.
"It is not what you want our president to be," Sen. Amy Klobuchar, D-Minn., said on Capitol Hill Thursday. "He is doing this to distract people. He's doing it to invigorate a band of people that support him. ... This is not leadership. This is using people as political pawns."
Sen. Kamala Harris, D-Calif., one of two women of color running for president, said, "The chant was created not by the crowd, but by the president's tweets."
"It is clearly not a sign of real leadership," Harris added. "Contrast it with a real American leader like John McCain, who during the campaign in 2008, he stood up, he spoke up. ... He understood as an American hero that the voice of someone who wants to be, much less is the president of the United States, must be a voice that is about elevating discourse, that is about speaking to our better selves."
Former Vice President Joe Biden, who said in the announcement video launching his candidacy, "We are in the battle for the soul of this nation," condemned the president's remarks speaking to reporters in Los Angeles on Wednesday.
"This is a game. This about dividing the country. This is about dividing and raising the issue of racism across the country. Because that's his base. That's what he's pushing," he said.
"Mr. President," Biden added, "condemn, outright condemn, those folks that came out of those fields. Outright condemn the Ku Klux Klan. Outright condemn white supremacists. Let me hear you say, 'I condemn them.'"
Washington Gov. Jay Inslee said on MSNBC that being an American citizen is an honor, and said he believes there's a responsibility that comes with that to protect "the most treasured thing we have," which is "building a new nation every day with people from around the world."
"We should continue that tradition," he added.
Before taking off North Carolina Wednesday afternoon, Trump asserted to reporters he was "winning the political fight" in his battle with the four progressive Democrats, because he claimed they are not "espousing the views our country."
"We have never seen a President use this kind of racist language in modern times, and the only other presidential candidate to talk even closely in these terms was George Wallace," said ABC News political analyst Matthew Dowd said, reiterating Booker.
Dowd, though, said the president's rhetoric benefits the Democrats vying to take on Trump in 2020, and those in Congress.
"It allows them to consolidate their base voters with solid support behind them, and it offends the swing voters who will be key for the presidential and congressional elections in 2020," Dowd added. "Absent a deteriorating economy, Trump's base of 40% is solidly behind him, but he continues to not expand to voters above that, which is problematic in 2020."
Earlier on Thursday, Biden said the Democratic nominee must "be the opposite of what (Trump) is."
"I said when I announced my candidacy," Biden added, "that I was running to restore the soul of this country, and I was being literal, not figurative."
ABC News' Rachel Scott, Will Steakin, Molly Nagle and Christopher Donato contributed to this report.