Not that long ago, they were just a slender fraction of the party, one kept at arm’s length by presidential candidates. But today, black voters have emerged as a muscular political force and one of the most intensely courted constituencies in Democratic politics.

In 2020, they are likely to account for at least one out of every four ballots cast in the party's presidential primaries, more than tripling — and perhaps even quadrupling — the share they accounted for just a few decades ago.

It’s a political and demographic revolution over the course of 40 years that we are able to document here through exit polling, which major media organizations have been sponsoring on a wide scale in every Democratic presidential primary race since 1976.

But until now, much of this data has been hard to come by, unavailable online, walled off in academic archives, even discarded by the news media outlets that sponsored it.

But thanks to the assistance of William Mayer, a political scientist at Northeastern University and an expert on presidential campaigns, NBC News has assembled for the first time a publicly available state-by-state record of the black vote for each of the nine competitive national Democratic campaigns since the inception of widespread exit polling. (Read about our methodology here.)

It begins in 1976, when the Voting Rights Act was barely a decade old, all-white-candidate fields were the norm, and the ties between African Americans and the Democratic Party were strained. And it extends through the 2016 campaign, by which point that bond had strengthened and sealed, all while a broader reshuffling had pushed older and blue-collar white voters toward the GOP and left Democrats more reliant than ever on support from nonwhite voters.

Democrats Sargent Shriver, Morris Udall, Fred Harris, Jimmy Carter, Henry Jackson, Birch Bayh and Milton Shapp at a forum in Boston on the eve of the New Hampshire primary on Feb. 24, 1976. AP file

In 2016, African Americans made up 24 percent of Democratic primary voters, the most ever. And that share is expected to climb in 2020, especially with the presence of two major black candidates, and could be decisive in determining the party’s nominee.

In a nod to the crucial role black voters now play in the party, the Democratic National Committee has once again designated South Carolina’s primary as one of four leadoff contests. In 2016, the state’s Democratic primary electorate was 61 percent black.

2020, then, presents a range of questions when it comes to black voters:

Will they, as in 2016 with Hillary Clinton and ’08 with Barack Obama, coalesce behind a single candidate?

To what degree will they be motivated to back a black candidate?

And if that is a significant factor, which black candidate is best positioned to benefit?

Will divisions within the black electorate emerge, based on age, gender, geography or other factors?

Will any of the nonblack candidates attract significant support from African American voters?

Could Joe Biden’s decades of relationship-building with black party leaders, along with memories of his partnership with Obama, translate into endorsements and grassroots support?

The data we are presenting here is from exit polls that were conducted for various media organizations in individual state primaries. Especially in the older contests, the data was often gathered for CBS News and The New York Times, which were particularly aggressive at the dawn of the exit polling era. But ABC News and NBC News also sponsored some early exit polls, and since 1992 state primary exit polling has been organized by a consortium of media outlets.

The data picks up with the 1976 campaign. While black voters had been siding with Democratic presidential candidates since at least the days of Franklin D. Roosevelt (with their allegiance reaching new heights during Lyndon Johnson's presidency), the depth of their loyalty remained an open question.

Black politicians were winning office in larger numbers — a handful in Congress, some in big city mayoralties and more in state legislatures, particularly in the South. Many weren’t sure if the Democratic Party was the right vehicle for their ambitions, and for the black community’s broader aspirations. Julian Bond, a Georgia state senator and civil rights veteran, considered an independent presidential campaign in 1976. Two years later, Jesse Jackson, another product of the civil rights movement, addressed a meeting of the Republican National Committee and declared the black vote up for grabs — if the GOP would make the effort.

The ’76 campaign played out as debates over busing and fair housing roiled neighborhoods in the North. The rising black constituency posed a strategic dilemma for the all-white roster of Democratic candidates, which weighed outreach against fears of a backlash from blue-collar “white ethnics.” The mere act of campaigning in black areas was enough to win Jimmy Carter credit from the top-ranking black official at the Democratic National Committee, who said: “He isn’t saying much, but he’s going.”

That wouldn’t do for long, though, and the Carter presidency proved a turning point. From black leaders, Carter faced accusations of reneging on commitments and taking their support for granted. His 1979 decision to dismiss his most prominent black ally, Andrew Young, as U.N. ambassador inflamed the situation and helped convince some black leaders to back Sen. Ted Kennedy’s primary challenge the next year. But Kennedy’s campaign created frustrations of its own, and ended up feeding the energy that gave rise to what would prove to be a major breakthrough in black politics: Jackson’s 1984 presidential campaign.