The dark politics of anger, division and fear were on display in campaigns across the country this year, as Republican candidates for Congress and governor — and fringe groups who support them — embraced the racially inflammatory brand of politics that Mr. Trump unleashed in 2016. With the presidential campaign of 2020 effectively underway on Wednesday, there is little reason to think Mr. Trump will back away from a tactic that clearly rallies his base.

And as he showed his party in the closing days of the midterms, there may be no way for Republicans to escape his shadow.

“He’s tapped into something really powerful and really ugly in the American electorate,” said Julian E. Zelizer, a historian at Princeton University, “and it’s going to be hard to put that back into the bottle.” ...

Racially charged messaging is not new. Ian F. Haney López, a University of California, Berkeley, professor who wrote the book “Dog Whistle Politics,” says the 2018 campaign is the culmination of a more than “50-year pattern,” dating back to the presidential campaign of Barry Goldwater, a Republican. President Richard M. Nixon’s “Southern strategy” courted white segregationists and effectively drove African-Americans to the Democratic Party.

Other presidential candidates have since dabbled in racial politics: Ronald Reagan lauded states’ rights in a 1980 speech in Neshoba County, Miss. In 1988, when George Bush was running for the White House against Michael Dukakis, Bush backers ran an ad spotlighting Willie Horton, a black convicted murderer who raped a white woman while furloughed from prison during Mr. Dukakis’s time as governor.

But, Mr. Zelizer noted, Mr. Reagan backed away from the speech and Mr. Bush distanced himself from the ad when each man faced criticism. By contrast, he said, “what President Trump keeps doing is, each time there is criticism, he comes right back in more aggressive fashion.”