The Confederate flag flies near the South Carolina Statehouse on June 19 in Columbia, S.C. (AP Photo/Rainier Ehrhardt)

Even if candidates don’t talk about it, race will be among the most important factors in the outcome of the 2016 presidential election. The Democratic Party has the support of 56 percent of Latinos, 65 percent of Asian Americans and 80 percent of the black population, according to the Pew Research Center. The share that supports the Republican Party is disproportionately white.

For decades, those racial divisions have shaped how individual candidates expect to perform at the polls. If Republican candidates were as popular among minority voters as they are among white voters, winning elections would be much easier. And Democrats would dominate the polls today if they had maintained the support of the white electorate.

They didn't. Democrats began losing the support of white voters after World War II, particularly in the South. During the civil rights movement, white Southerners left the Democratic Party in droves.

Some scholars have argued that changes in the South’s economy caused the party's decline there. Since wealthier voters tend to be more conservative, it's plausible that Southerners' move to the Republican Party is a reflection of the region's economic growth.

Other historians, though, have long argued that civil rights legislation supported by President Kennedy and other Washington Democrats led to the party's loss of power in the South. And a study published this month by the National Bureau of Economic Research, based on more than half a century of newly available polling data, supports that interpretation. It was the issue of race, the review of Gallup archives shows, that undid Democrats in the former Confederacy.

Birmingham, 1963

Economists Ilyana Kuziemko of Princeton University and Ebonya Washington of Yale University are among the first to analyze Gallup’s data. Their study looks back to 1963, when activists boycotted segregated businesses and public facilities in Birmingham, Ala. In June, two black students tried to enter a building at the University of Alabama in Tuscaloosa as Gov. George Wallace, a Democrat, stood in the doorway to block them. President Kennedy, also a Democrat, responded with a show of force, summoning the National Guard. Wallace gave way, and a few hours later, the president went on television to call for equality.

"One hundred years of delay have passed since President Lincoln freed the slaves, yet their heirs, their grandsons, are not fully free," Kennedy said. "They are not yet freed from the bonds of injustice. They are not yet freed from social and economic oppression."

[Read more: The most racist places in America, according to Google]

Kennedy's white constituents in the South were outraged. They had overwhelmingly supported Democrats since the end of Reconstruction. Yet between April 5 and June 23 of that year, the share of white Southerners who approved of Kennedy declined by 35 percentage points to about 20 percent , according to the Gallup polls. This abrupt shift marks when white Southerners turned against Democrats, Kuziemko and Washington argue. The percentage of whites in the rest of the country saying they approved of the president did not change appreciably.

At the end of the World War II, nearly 80 percent of white Southerners were Democrats, compared to 40 percent of whites in the rest of the country. By the Reagan administration, white Southerners were no more likely to identify as Democrats than whites elsewhere. Today, the white vote in the South is almost solidly Republican.

The question of race

For decades, Gallup has been asking Americans whether they would consider voting for a black presidential candidate. White Southerners who said they would were no less likely to call themselves Democrats after the spring of 1963 than before. But many of those who said they wouldn’t vote for a black candidate left the party.

That was compelling evidence that race had everything to do with Democrats' decline in the South, but the two economists wanted to rule out other possible explanations. Maybe Democrats who held conservative views on other issues also happened to be more conservative on race. If so, the decline in the number of white southern Democrats might have resulted from the party's positions on other issues. At that time, President Lyndon B. Johnson was pushing his “Great Society” vision, launching a “war on poverty” and creating Medicare in 1965. Those positions also might have alienated more conservative Democrats.

But if racially conservative white Democrats in the South also were more conservative on economic issues and health care, their counterparts in other parts of the country presumably would be, too. Conservative Democrats west of the Mississippi and north of the Mason-Dixon Line would have left the party as it shifted to the left on economic issues, regardless of their views on race. But they didn’t. Whites outside of the South who wouldn’t vote for a black president were no less likely to identify as a Democrat after the civil rights legislation than before.

And Kuziemko and Washington did not find that white Southerners with conservative views on race were more likely to hold conservative views on other issues, whether compared to other white Southerners or compared to racially conservative white voters in other parts of the country. The researchers cite the American National Election Studies, which showed little regional variation in white voters' views on taxes, big business, housing, health insurance and more before the civil rights movement.

[Read more: Why the South is the worst place to live in the U.S. — in 10 charts]

In other words, it's hard to see why white voters in the South unwilling to vote for a black presidential candidate would have left the party because of its positions on health care and other issues, while similar Democrats in the rest of the country remained loyal partisans.

The researchers also considered whether those Southerners who opposed a black president did so not because of racial prejudice, but because of their support for states' rights. Gallup asked respondents if they would consider voting for a black presidential candidate in their party. Because the Democratic Party was more racially diverse in the North, Southern Democrats might have assumed that a black Democratic nominee would be a Northerner, endangering the South's control over its own affairs.

But Gallup also asked respondents whether they would be willing to vote for a hypothetical Catholic, Jewish or female presidential candidate -- all of whom would likely have come from a Northern state -- and white Southern Democrats who opposed candidates in these categories were not any more likely to leave the party. Race, rather than states' rights or other political issues, seems to have caused the demise of the Southern white Democrat.

The Southern strategy

Even when policies are not as explicitly racial as the civil rights legislation, race has played a role in political allegiances. Lee Atwater, an adviser to President Reagan, discussed the Republican Party’s “Southern strategy” in an interview in 1981, explaining that some white voters might subconsciously support conservative policies apparently unrelated to race, if those policies had different consequences for different races.

"You say stuff like forced busing, states' rights and all that stuff and you get so abstract," Atwater said. "You talk about cutting taxes and these things you're talking about are totally economic things and a byproduct of them is, blacks get hurt worse than whites."

More than two decades later, Republican National Committee Chairman Ken Mehlman criticized that strategy.

"Some Republicans gave up on winning the African American vote, looking the other way or trying to benefit politically from racial polarization," he said in 2004. "I am here today as the Republican chairman to tell you we were wrong."

Yet research suggests the racial undertones of policy debates continue to affect some Americans’ political affiliations. A study published in 2013 concluded that white respondents to an online survey were more likely to identify with the tea party if they held anti-black sentiments.

In this election cycle, though, race is not just an undertone. Politicians are debating racial disparities in incarceration and the deaths of unarmed African Americans at the hands of police. Americans haven't talked about the issue so openly for decades, and it remains to be seen how this new discussion about race will color the electoral map.

More from Wonkblog:

A poverty map where the South looks the best

What the debate over the Confederate Flag and states’ rights really means

The economy has done better under Democrats