Fears that the 2020 election campaign will be marked by violence are growing after a raucous crowd at a Trump rally chanted “send her back” about Rep. Ilhan Omar Ilhan OmarOmar fires back at Trump over rally remarks: 'This is my country' Trump attacks Omar for criticizing US: 'How did you do where you came from?' Democrats scramble on COVID-19 relief amid division, Trump surprise MORE (D-Minn.), one of four congresswomen attacked earlier in the week by the president.

Rep. Justin Amash Justin AmashInternal Democratic poll shows tight race in contest to replace Amash Centrist Democrats 'strongly considering' discharge petition on GOP PPP bill On The Trail: How Nancy Pelosi could improbably become president MORE (I-Mich.), a conservative who recently left the Republican Party in protest over the president’s behavior, tweeted on Thursday morning that the Trump rally the previous night was “how history’s worst episodes begin.”

Rep. Alexandria Ocasio-Cortez Alexandria Ocasio-CortezOn The Money: Anxious Democrats push for vote on COVID-19 aid | Pelosi, Mnuchin ready to restart talks | Weekly jobless claims increase | Senate treads close to shutdown deadline McCarthy says there will be a peaceful transition if Biden wins Anxious Democrats amp up pressure for vote on COVID-19 aid MORE (D-N.Y.), one of the members of the self-proclaimed “squad” that Trump has targeted, told reporters on Thursday that the president had “put millions of Americans in danger” with his language.

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The crowd’s chants for the expulsion of a congresswoman from the country on Wednesday evening raised concerns over the nation’s febrile political mood to a new level.

Democrats in Congress called for additional security for Omar and other rank-and-file members, who have seen a spike in death threats according to Capitol Police.

“These are dangerous times. Every member of this House needs additional security,” said Rep. Al Green Alexander (Al) N. GreenThe Memo: Trump's race tactics fall flat Trump administration ending support for 7 Texas testing sites as coronavirus cases spike The Hill's Coronavirus Report: Miami mayor worries about suicide and domestic violence rise; Trump-governor debate intensifies MORE (D-Texas).

On Thursday, Trump claimed that he “was not happy” with the anti-Omar chant in North Carolina. Asked why he did not say anything to stop it, Trump demurred, claiming that he “started speaking very quickly” after the chant broke out.

He did not do so, instead pausing for well over 10 seconds.

Trump ignited the broader controversy on Sunday by tweeting that the congresswomen in question should “go back” to the “crime-infested places from which they came.”

All four congresswomen — Ocasio-Cortez, Omar and Reps. Ayanna Pressley Ayanna PressleyFauci, Black Lives Matter founders included on Time's 100 Most Influential People list Trump attacks Omar for criticizing US: 'How did you do where you came from?' Pressley applauded on House floor after moving speech on living with alopecia MORE (D-Mass.) and Rashida Tlaib Rashida Harbi TlaibTrump attacks Omar for criticizing US: 'How did you do where you came from?' George Conway: 'Trump is like a practical joke that got out of hand' Pelosi endorses Kennedy in Massachusetts Senate primary challenge MORE (D-Mich.) — are U.S. citizens. Only Omar, a child refugee from Somalia, was born outside the United States.

Trump and his allies have denied that his tweets and comments are racist.

But the president has also signaled he is intent on using white identity politics as a central part of his appeal, meaning that with almost 16 months left until Election Day this is plenty of opportunity for fuel to be added to the fire.

Experts on political extremism express concern that the president’s words could lead to violence.

“This is the president of the United States — arguably the most powerful person in the world — and so it is hard to overestimate how dangerous it really is,” said Peter Simi, a professor at Chapman University who specializes in the study of extremism.

Referring to Trump’s rally, he added, “Most people who hear those words, even if they agree, are not going to act in a violent fashion. But it only takes one person who has some vulnerability, some susceptibility [to violence] and tragic events could occur.”

David Niewert, the author of “Alt-America,” a book that examines the radical right in the Trump era, drew attention to previous examples where people sympathetic to the president have acted violently.

The most high-profile example is likely Cesar Sayoc, who has pleaded guilty to sending pipe bombs in the mail to a number of high-profile Democrats, including former President Obama, and Trump critics.

In April, Sayoc wrote to the judge in his case that attending a Trump rally “became like a new found drug” for him.

Neiwert insisted that no other president, with the possible exception of President Andrew Jackson in the 19th century, was so willing to indulge in demagoguery as Trump.

“I don’t think we’ve ever seen in history a president who explicitly is attempting to divide the country rather than unify it. And that is really a horrifying and frightening thought,” he said.

Trump supporters — and conservatives more generally — object to this analysis.

The president has cast his attacks on the congresswomen as pushing back against Democrats who use what he characterizes as “anti-USA” rhetoric.

Extremist violence has not always been in one direction, either. House Minority Whip Steve Scalise Stephen (Steve) Joseph ScaliseHouse GOP slated to unveil agenda ahead of election House panel details 'serious' concerns around Florida, Georgia, Texas, Wisconsin elections Scalise hit with ethics complaint over doctored Barkan video MORE (R-La.) was badly injured in a June 2017 shooting at a baseball practice, carried out by a left-wing activist.

Trump, despite claiming to be unhappy about the “send her back” chant on Wednesday, has also said that the people saying those words are “people who love our country.”

Later Thursday, after CNN anchor Jake Tapper Jacob (Jake) Paul TapperThe media's misleading use of COVID-19 data Julia Louis-Dreyfus: 'We can't spend much time grieving' Ginsburg Pence aide dismisses concerns rushed vote on Trump nominee will hurt vulnerable senators MORE accused Trump’s reelection campaign of using “naked racism” as part of its strategy, a campaign spokesperson blasted the claim as “ridiculous.”

Experts in extremism are alarmed both by how Trump is willing to transgress boundaries that other presidents have respected — and because it may work for him in electoral terms.

Trump’s 2016 campaign ignited several race-related controversies, including the candidate saying in his launch speech that Mexico was sending “rapists” to the United States.

Trump also called during that campaign for the “shutdown” of Muslim immigration for an unspecified period.

Heidi Beirich of the Southern Poverty Law Center said that Trump was now engaging in “a calculated strategy to embolden parts of his base.”

She added that “to stop this, to unwind this, to once again make racism unacceptable, is going to take some work. You have to think that one of the things that might do it is if people start losing elections by doing this.”

But there is a long time to go before Election Day 2020.

Trump appears to be trying to tie the Democratic Party nationally to the members of “the squad.”

There are also several nonwhite candidates running for the Democratic nomination, and if one of them were to become his general election opponent — at the moment, Sen, Kamala Harris Kamala HarrisHundreds of lawyers from nation's oldest African American sorority join effort to fight voter suppression Biden picks up endorsement from progressive climate group 350 Action 3 reasons why Biden is misreading the politics of court packing MORE (D-Calif.) appears the most likely possibility — racial issues could become even more intense.

Experts point out that even the election is not necessarily the end of the story.

Simi, the Chapman University professor, said that for the most hard-line of Trump supporters “victory will be a further emboldening and defeat is going to be framed as some kind of fraudulent election.”

Asked if he saw any reason for optimism in the immediate future, he replied, “I try, for the sake of principle, to find optimism. But right now there is not a lot of reason for it.

“We are in a very dark time.”