(CNN) President Donald Trump's strategy in 2020, similar to 2016, is to use race to divide the country. There is no mystery in what he's doing because he's done it before.

Pinching the racial nerve of US society is behind his attacks on Democratic Rep. Elijah Cummings of Maryland and the whole city of Baltimore as well as his efforts to vilify four progressive lawmakers -- all women and minorities.

This latest episode involves Cummings, chair of the House Oversight Committee, who is overseeing multiple investigations of Trump and top administration officials. Trump tweeted over the weekend that Cummings' district is a "disgusting, rat and rodent infested mess."

That came two weeks after his controversial tweet that the progressive lawmakers should "go back and help fix the totally broken and crime infested places from which they came."

Each of Trump's targets is a congressperson of color. One of them is a naturalized citizen. There is no question of the ugly racial subtext in Trump's tweets. The elements of these attacks repeat on a theme: explicitly connecting his nonwhite opponents to crime and social decay.

Any complaint about Baltimore or any other US city would require a careful examination of the country's history of unequal housing policies and investment. And it should be noted that Democrats are having their own internal debate about what government can do to address generations of racial inequality, either by studying a form of reparations for the descendants of slaves or creating new benefits for the neediest Americans. But the specifics of Baltimore and social policy are beside the point.

The audience for these attacks wasn't voters in Baltimore or anywhere near it, just as the audience for Trump's vicious attacks on immigrants isn't people who live in border areas or cities. It's the base of supporters in the South and in the Rust Belt who handed him electoral victory in 2016 and who he hopes will do so again.

"He is exciting his base," said Andra Gillespie, a political scientist at Emory who is an expert on race and politics. "He's reminding folks based on their resentments, stoking their resentments, that there are all these testy people who are saying we should do this or we should do that and they shouldn't have this much power.

Exploiting fear is nothing new for him. Trump accepted his party's nomination in 2016 describing the US as a nation in "crisis" and promising to restore law and order to Make America Great Again.

Trump vs. the GOP's Southern strategy

Trump's efforts to divide the country have drawn some comparisons to Richard Nixon's strategy in 1968 to make Republicans out of Southern Democrats frustrated by the civil rights movement and the Great Society efforts signed into law by President Lyndon B. Johnson.

But Timothy Naftali, a Nixon scholar at New York University, said the comparison is not entirely correct.

Nixon's effort was built on specific research and the belief by Nixon aides, as The New York Times reported in 1970 , that elections could be won and coalitions built on racial enmity. Nixon was trying to make the Republican Party bigger by adding Southern voters. Trump is distilling the GOP down to his own base.

"The Nixon Southern strategy was an initiative to tear away a large percentage of the Democratic base in order to make the Republican Party a majority party," said Naftali. "Donald Trump's strategy of intolerance is designed to bring nonpolitical people into the process because he does not want to reach out beyond his base."

But there is a direct line from Nixon's coded "law and order" campaign in 1968 to Ronald Reagan's warnings against " welfare queens " in 1980 and beyond, according to a recent piece by Angie Maxwell , a University of Arkansas political scientist.

"Trump, in many ways, is no anomaly. He is the very culmination of the GOP's long Southern strategy," she wrote.

Nixon's coded language might have felt like the middle road in 1968, when the avowed segregationist and once and future Democrat George Wallace actually won five Southern states as an independent pushing states' rights and racial separation. Wallace ran for president again in 1972 as a Democrat, focusing on opposition to school busing, but a would-be assassin's bullet stopped his primary campaign.

Trump, more than 50 years later, is not pushing anything close to segregation. But he is appealing to a distinct base of white men similar to Wallace's. Look back at his Charlottesville comments in 2017, when he said there were "fine people" among the white nationalists who marched through town, one of whom killed a counter-protester when he drove his car into the crowd.

"Nixon used dog whistles to encourage Southern Democrats to become Republican. Donald Trump is using a bullhorn. He is not trying to build a new coalition," Naftali said. "What he's doing is trying to transform the Republican Party into a Trumpist party."

Trump's GOP is not apologizing

He has certainly derailed the GOP from a path toward attempts at reconciliation with the African American community.

Ken Mehlman, who was Republican National Committee chairman during the George W. Bush administration, gave a speech at the NAACP that year apologizing for his party's past exploitation of racial divides in the country but arguing that Republicans could be the party of the future for black Americans after decades of not reaching out.

"Some Republicans gave up on winning the African American vote, looking the other way or trying to benefit politically from racial polarization," Mehlman said. "I am here today as the Republican chairman to tell you we were wrong."

Mehlman declined to be interviewed for this report, but his sentiment of atonement feels a very, very long way from Trump's rhetoric of division. And the lack of public objection to Trump's efforts by elected Republicans today demonstrates how the party has gravitated toward him.

The big question of 2020 is whether such a base campaign can again win Trump enough electoral votes to defeat a Democrat.

There is evidence that his efforts to further divide the country are working.

In a Pew study released in April , 58% of Americans said race relations in the US were bad, 56% said Trump has made them worse and nearly two-thirds said people are expressing racist views more with Trump as President.

Black Americans have a much dimmer view of the situation; half say the US will never reach racial equality. The poll documents a similar partisan divide on racial issues and Trump's role in making things worse or not.

There's absolutely no indication Trump will stop this type of attack, since he's not paying any sort of price for it.

"Ultimately what it's going to take is the righteous indignation of a critical mass of Republicans who are willing to stand up and tell the President this is not acceptable," said Gillespie.

There are a few lonely Republican voices frustrated by his rhetoric. Maryland Gov. Larry Hogan, a Republican, and former RNC Chairman Michael Steele have both criticized Trump's attacks on Baltimore and Cummings

Former Rep. Mia Love of Utah, now a CNN commentator, was one of two African American Republicans in the House until 2019. She became one of the few Republicans to call Trump out for his recent tweets aimed at Cummings and Baltimore during an appearance on CNN's "State of the Union."

"In the recent tweets and the things that I have seen, enough is enough," she said. "It really is disheartening. The greatest threat that we face is the division of America, and I don't believe that any president should be playing a part in dividing America."

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She later added that all Republicans should not be lumped in with Trump's efforts.

"I want to remind everyone that Republicans are not racist," she said. "They are not. The policies we believe in are good for everybody."

But former Michigan Gov. Jennifer Granholm, a Democrat appearing alongside Love, pointed out that sitting Republican lawmakers have not broken with Trump's words.

"The problem is when they don't -- when elected Republicans do not stand up against these racist tweets, they are aiding and abetting a racist President," Granholm said.

As the country prepares for another presidential campaign, it's worth noting that Trump is not the only force who has attempted to exploit racial divisions. Russian internet trolls who were trying to help Trump win in 2016 focused much of their efforts on racial divides in the US, as documented by CNN and others.

That foreign governments try to weaponize these internal divides should be a cause of concern that unifies Americans, but the opposite is occurring. By targeting certain Americans and certain cities, Trump makes it feel as though he's not even trying to be everyone's President. And that's the point. That's his whole strategy.