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(CNN) —  

In a moment of unrestrained demagoguery, President Donald Trump presided Wednesday over a crowd chanting “Send her back! Send her back!” about an American Muslim congresswoman who he targeted with racist attacks.

The scenes at a North Carolina rally provided an ugly overture to a 2020 election campaign already soaked in hate. They exemplified the tribal politics and white nationalism that Trump is making clear he plans to ride to reelection, no matter their impact on America’s fragile societal harmony.

The chants of “Send her back!” referred to Somalia-born, American citizen Rep. Ilhan Omar of Minnesota, one of four minority lawmakers attacked by Trump over the weekend. The invective from the crowd replaced the “Lock her up!” and “Build the wall!” chants of Trump’s first campaign with a jarring racial refrain that the commander-in-chief, speaking from behind a podium bearing the symbolic presidential seal, made no effort to stop.

Reinforcing days of attacks on the four progressive Democratic women, known as “The Squad” on Capitol Hill, Trump questioned their patriotism and highlighted some of their controversial comments on issues including Israel, law enforcement and the September 11th terrorist attacks, in some cases distorting their records.

“They don’t love our country. They are so angry,” Trump said, lambasting the group as “hate filled extremists.”

“If they don’t like it let them leave, let them leave,” Trump said.

Other presidents, like Richard Nixon for instance, have used veiled racial messaging in elections. But there is no modern equivalent of a US president so openly encouraging racial and tribal undercurrents on the campaign trail in order to consolidate his own power.

“We’ve had racist presidents,” CNN presidential historian Timothy Naftali said on “Anderson Cooper 360.”

“But they did not express their racism as head of state, the way that Donald Trump through his actions – most importantly the tweets the other day and how he responded to Charlottesville – the way that President Trump has done.”

Democratic presidential candidates reacted to the menacing scenes by saying they showed why Trump must be driven from office.

“It’s vile. It’s cowardly. It’s xenophobic. It’s racist. It defiles the office of the President,” California Sen. Kamala Harris wrote on Twitter.

Former Vice President Joe Biden tweeted, “Mr President, I am here to tell you this. This is OUR country. The United States of America. You’ll never understand what makes us strong.”

Even Rep. Adam Kinzinger, an Illinois Republican, tweeted Thursday that the chant “would send chills down the spines of our Founding Fathers” and said “this ugliness must end.”

The President was making his first appearance on the trail since escalating his attempt to make the group of congresswomen of color, who are well to the left of most liberals and who in some cases have their own history of inflammatory rhetoric – a proxy for the entire Democratic Party.

The showdown is revealing the central question of the 2020 election campaign. Will Trump’s embrace of searing racial grievances, tough immigration commentary and rhetorical targeting of non-white adversaries that helped electrify his mainly white, blue-collar base and win the White House in 2016 deliver him a second term?

Or will the tactic drive away some moderate, suburban voters around cities such as Philadelphia, Detroit and Milwaukee who he needs to return to his fold to keep a similarly narrow path to victory open in 2020? And while it is likely to solidify Trump’s base, will such an approach ignite record breaking Democratic turnout?

Evidence from the midterm elections in 2018 suggests that Trump’s scorching immigration rhetoric worked in deep red Republican states. But it backfired elsewhere as Republicans lost the House. According to CNN exit polls, only 23% of respondents who voted Democratic said immigration was their greatest concern. And health care, on which Democrats based their campaign, was the defining issue of the election and for 41% of voters.

Still, though the midterms were a referendum on Trump, he wasn’t on the ballot – a factor that means that the lessons of 2018 cannot be simply transferred to 2020.

And there is another variable in 2020 that could be decisive: Hillary Clinton will not be involved. The Democratic nominee was a hugely polarizing force as she carried her own immense political baggage and as well as that of a pair of two-term Democratic presidents – Bill Clinton and Barack Obama – on her back.

The President lost the popular vote and only carried the three crucial swing states – Michigan, Pennsylvania and Wisconsin – by a combined total of less tha