(CNN) An escalating cycle of action and reaction between President Donald Trump and the Democratic presidential contenders is thrusting issues of race relations and American identity to the center of the developing 2020 campaign .

In office, Trump has appealed even more overtly than he did as a candidate in 2016 to the racial resentments of some white voters with such comments as his call for four House Democratic women of color to "go back" where they came from and his 2017 declaration that "very fine people" had marched on both sides in Charlottesville, Virginia.

In response partly to Trump, and partly to demands from the party base, the 2020 Democratic presidential contenders are talking more explicitly than Hillary Clinton did in 2016 about "systemic racism" (Sen. Elizabeth Warren of Massachusetts and South Bend, Indiana, Mayor Pete Buttigieg), "institutional racism" (Sen. Kamala Harris of California) and "institutional segregation" (former Vice President Joe Biden).

At last week's Democratic presidential debate in Houston , former Rep. Beto O'Rourke of Texas echoed a recent New York Times project and declared that the introduction of slavery in 1619, not the Declaration of Independence in 1776, was the "foundational" event in shaping America. "Racism in America is endemic," O'Rourke declared. "It is foundational. We can mark the creation of this country not at the Fourth of July, 1776, but August 20, 1619, when the first kidnapped African was brought to this country against his will."

Simultaneously, the eruption of new allegations of sexual assault and misconduct against Supreme Court Justice Brett Kavanaugh -- and Trump's emphatic immediate defense of his nominee -- is but one of several issues that are likely to highlight questions about the changing role of women in society.

The cumulative effect could be to compound the dynamic evident in 2016, when an array of political science studies found that attitudes about racism and sexism predicted support for Trump or Clinton much more powerfully than people's assessments of their own economic situations.

In 2016, the studies found, the more likely voters were to say that racism and sexism are no longer problems in American life, the more likely they were to vote for Trump. Recent surveys have likewise found that Trump runs far better among voters who believe that discrimination against minorities is not a major problem than those who do; Trump supporters are also far more likely than other Americans to maintain that discrimination against whites is a significant problem, the surveys have found.

Attitudes toward these dynamics "is something that has divided the parties for quite awhile," says Brian Schaffner, a political scientist at Tufts University who conducted some of the research about the impact of attitudes toward racial and gender discrimination in 2016. "But there's something Trump has added to this ... Trump and his rhetoric just make these divisions more explicit."

Rhetoric is raising issues' profile

In 2020, the contrast between the nominees on issues of race relations will likely be as stark as any since the Republicans in 1964 nominated Sen. Barry Goldwater, who opposed the Civil Rights Act, against President Lyndon Johnson, who steered it into law. Trump has appealed to racial resentments more openly than any national figure in either party since George Wallace in the 1960s, while declaring that Democrats are the real racists for accusing him, and by implication his supporters, of racism.

At the same time, the Democratic candidates are portraying America as infected with racism more openly and insistently than any previous party leaders, even Barack Obama, the first African American president. Many of the Democrats are explicitly calling Trump a racist and some have expressed sympathy for the concept of providing reparations to African Americans for the long-term impact of slavery.

That means both parties are now framing the debate over America's changing identity in a manner that seems likely to elevate the role of these issues in sorting voters between the two sides.

Candidates Elizabeth Warren, Kamala Harris and Andrew Yang during the Democratic presidential debate on September 12, 2019, in Houston.

Schaffner thinks these divisions could grow more pronounced in 2020 whether Democrats nominate a woman or racial minority to face Trump, or pick a white man.

If Democrats select a woman or racial minority, that candidate, like Clinton and Obama, will personify social change in a way likely to provoke the core Republican voters who are most uneasy about it, he notes. On the other hand, if Democrats nominate a white man, that candidate will likely feel pressure to respond to the party's increasingly diverse electoral coalition by talking more explicitly than past nominees about "structural" or "systemic" racism.

"If Democrats nominate a white male, that person on one hand might seem less threatening to moderate voters who maybe have somewhat conservative racial views," says Schaffner. "On the other hand, that candidate, given what the Democratic Party base looks like these days, is going to have to campaign somewhat more explicitly on issues that matter to minority groups."

That dynamic was evident in the sort of charged language that O'Rourke used at last week's Democratic debate about slavery's role in shaping American society. Major components of the Democratic coalition might consider such bracing talk long overdue, but it is also virtually certain to make "racially conservative voters feel more resentful," notes Schaffner.

Attitudes have bigger role in shaping votes

The heightened pressure on the Democratic field is also evident in the fierce blowback from many liberal nonwhite writers and political activists to Biden's comments during last week's debate about sending social workers to help low-income parents raise their children (in part by using a "record player" at night).

Detailed studies of public opinion during the Trump era have almost universally concluded that there has not been significant change in the share of Americans who believe or dispute that discrimination against minorities and women is still a problem in American society. What's changed under Trump is that attitudes on those questions have become more important in shaping how Americans vote.

In their 2018 book "Identity Crisis," for instance, political scientists John Sides, Michael Tesler and Lynn Vavreck reported that multiple post-election surveys found that "whites' attitudes about race, ethnicity and religion came to play a larger role in 2016 than in other recent elections."

Working with two colleagues, Schaffner likewise found that in 2016 the more likely voters were to believe that racial discrimination is not a systemic problem and that women complaining about sexism were actually seeking unfair advantage over men, the more likely they were to support Trump. That relationship was as powerful among women voters as among men, and for both groups those attitudes dwarfed voters' assessments of whether their personal economic situations were improving or deteriorating.

In a follow-up paper this year , Schaffner found that attitudes about whether racism or sexism persists as a problem predicted support for Republican or Democratic congressional candidates in 2018 more powerfully than they did in 2016 or the previous midterm election, in 2014. The reason for that greater correlation was that some previously Republican-leaning voters who are more liberal on issues relating to racial and gender discrimination peeled away from the party to support Democratic candidates last year, he found.

An array of recent public opinion surveys has found that a huge chasm persists between the Democratic and Republican coalitions on whether minorities still face discrimination in America.

An August national Quinnipiac University survey found that voters who support Trump are much more likely than those who don't to believe that whites face significant discrimination in America today and African Americans and immigrants do not, according to detailed figures provided to me by the pollsters.

In the Quinnipiac survey, fully 57% of voters who approved of Trump's job performance said that discrimination against whites was a very or somewhat serious problem today, according to those previously unpublished results. Only 34% of voters who disapproved of Trump's performance agreed.

Conversely, while 94% of voters who disapprove of Trump's performance say discrimination against African Americans is a problem, just 41% of those who approve of him agree. The gap is similar for immigrants: While 95% of voters who disapprove of Trump say immigrants face discrimination, just 45% of Trump supporters agree.

Put another way, more Trump supporters say whites face discrimination (57%) than say immigrants (48%) or African Americans (41%) do. A solid majority of Trump supporters say African Americans do not face serious discrimination (55%) and a plurality say the same about immigrants (48%).

Among voters who don't approve of Trump, the pattern is inverted: Two-thirds of them say whites don't face discrimination, while only about 1 in 20 say the same about blacks or immigrants.

2020 polling reflects the trend

These attitudes loom over preliminary vote choices for 2020, according to the Quinnipiac findings. In the survey, Trump held a 49% to 44% lead over Biden among the roughly two-fifths of voters who believe that discrimination against whites is a problem; Biden led Trump by 64% to 28% among the majority who said it is not.

Conversely Trump held an 81% to 13% advantage among the quarter of the electorate who say discrimination against African Americans is not a problem, while Biden led by 70% to 22% among the nearly three-fourths who say it is.

Attitudes about gender relations divide Trump supporters and opponents in similar patterns.

An extensive recent study that pollster Tresa Undem conducted for the liberal women's activist group Supermajority found huge gaps between liberal and conservative women not only on political choices, such as whether abortion should remain legal, but on underlying social trends, such as whether women have achieved equal treatment in American society or whether women interpret too many remarks from men as sexist.

Undem's findings reinforced the results of studies from Schaffner and his colleagues, as well as those by Erin Cassese of the University of Delaware and Tiffany Barnes of the University of Kentucky , which found that female voters who supported Trump in 2016 were far less likely than those who backed Clinton to believe that women face discrimination and far more likely to believe children are better off if their mothers stay at home and that families succeed when men work and women raise the children.

For many Trump supporters, "they don't see a problem" with how women or minorities are treated, says Undem, whose firm polls mostly for nonprofit organizations and liberal advocacy groups. "Then, they are supersensitive to being called racist and sexist," she continues. "I just get the sense they have fused their own identity ... in Trump, so to attack Trump is to attack them. (They are saying,) 'No, I'm not a racist, and there's no racism.' "

Many polls this summer have found that a majority of Americans believe that Trump is racially insensitive and disrespectful; in the harshest possible phrasing, Quinnipiac found in July that a 51% majority of Americans consider Trump a "racist."

But private polling conducted in recent weeks by the firm GQR for a consortium of liberal groups has stirred debate in Democratic circles by concluding that the party nonetheless is unlikely to benefit among voters by directly labeling Trump a racist. That was partly because right-leaning voters consider the phrase too loosely applied to any conservative and partly because some swing voters recoil from the level of personal acrimony inherent in the charge.

Anna Greenberg, the Democratic pollster who conducted that research, says Democrats could find sturdier ground by accusing Trump of using race to divide the country -- rather than personally harboring racist beliefs -- and by talking about structural racial inequities.

"A wonky conversation about institutional racism and solutions to it I don't think is threatening to most people," she says. The tripwire for Democrats among some white voters, she believes, is when they frame policies to expand opportunities for minorities as a kind of penance for whites. "The difference is if you are saying, 'This is your fault and you must pay the price of your ancestors,' that's when I think you start getting in a little bit of trouble with voters."

Navigating that line isn't likely to be easy for the eventual Democratic nominee. That's especially true because Trump routinely portrays any criticism of his behavior as an attempt to brand his preponderantly white supporters as racist. From the other direction, as the fierce blowback against Biden's "record player" comments signal, Democrats face more pressure than ever to explicitly denounce racism and to portray it as the predominant force shaping life in America for minority groups, even if that formulation antagonizes some white voters.

Those dynamics are two trains, gathering speed, on track for a head-on collision before November 2020.