It was simple, specific and he put it plainly to a number of the top-tier 2020 Democratic presidential contenders who visited his National Action Network conference in New York this week.

Would they support a House bill, first introduced three decades ago, that would create a congressional panel to study the possibility of reparations for the descendants of slaves?

One by one, the candidates answered yes -- some not for the first time. In this 2020 primary, the issue of reparations, which for decades existed only on the fringes of mainstream political debate, has emerged as a litmus test for Democrats vying for a shot at unseating President Donald Trump next year.

Over the course of a few months, the candidates have wrestled with the question while liberal activists carried on a parallel debate over the definition of reparations, as they set out to create more concrete parameters for both the public and the candidates. Their consensus: H.R. 40, a piece of legislation handed off from former Michigan Rep. John Conyers, who left Congress in 2017, to Texas Rep. Sheila Jackson Lee, who is carrying the torch into this new Congress.

In an interview with New York Magazine , the writer Ta-Nehisi Coates, author of the Polk Award-winning essay "The Case for Reparations," said the candidates -- in the context of a political campaign -- should support H.R. 40.

"That's the bill that says you form a commission. You study what damage was done from slavery, and the legacy of slavery, and then you try to figure out the best ways to remedy it," Coates said. "It's pretty simple."

Democratic candidates, including Sens. Cory Booker and Elizabeth Warren as well as former Housing and Urban Development Secretary Julian Castro, have all addressed the reparations issue numerous times on the campaign trail in recent weeks.

During a CNN town hall in Jackson, Mississippi, in March, Warren argued that because of housing and employment discrimination, "We live in a world where the average white family has $100 (and) the average black family has about $5."

"So I believe it's time to start the national full-blown conversation about reparations in this country," Warren said. "And that means I support the bill in the House to appoint a congressional panel of experts, people that are studying this and talk about different ways we may be able to do it and make a report back to Congress, so that we can as a nation do what's right and begin to heal."

Booker, the senator from New Jersey, has expressed his frustration with the conversation around reparations -- stating that it is being "reduced to a box to check on a presidential list, when this is so much more of a serious conversation."

"Do I support legislation that is race-conscious about balancing the economic scales? Not only do I support it, but I have legislation that actually does it," Booker told CNN's Jake Tapper during a CNN town hall in Orangeburg, South Carolina, in late March.

During Sharpton's conference in New York this week, other contenders offered their full-throated support of H.R. 40, including South Bend Mayor Pete Buttigieg, former Maryland Rep. John Delaney, New York Sen. Kirsten Gillibrand, Minnesota Sen. Amy Klobuchar, former Texas Rep. Beto O'Rourke, Ohio Rep. Tim Ryan and Vermont Sen. Bernie Sanders.

When asked about reparations during an early March appearance on the popular radio show "The Breakfast Club," Sanders had seemed dismissive of the idea of "writing a check" to every African American: "Well, then there's a check to every Native American, who were nearly wiped out when the settlers first came here," Sanders told the "Breakfast Club" hosts. "I think the way we go forward is to build America together."

On Friday at NAN, Sanders said that if "the House and Senate pass that bill, of course I will sign it."

"There needs to be a study," Sanders said. "Let me also say this. I think what we need to do is to pay real attention to the most distressed communities in America. We have got to use 10% of all federal funds to make sure that kids who need it get the education, get the jobs, get the environmental protection that they need. And that would be a major focus of my efforts."

California Sen. Kamala Harris pledged to sign H.R. 40 during a Friday morning interview on the SiriusXM radio show "Make it Plain with Mark Thompson." Later, during her speech at the NAN convention, she added: "When I am elected president, I will sign that bill."

Former Colorado Gov. John Hickenlooper called slavery "the nagging unrelenting shame of America, that continues to deny the true promise of this country to too many of its citizens" during the NAN convention Friday.

"We must own our past, and acknowledge the shame, the sin, the injustice, and the ongoing consequences of enslaving an entire race of people," Hickenlooper said, endorsing the legislation. "And we must apologize and that apology should come from the Oval Office."

Endorsing Jackson Lee's proposal again this week, former HUD Secretary Castro noted that he has been talking on campaign trail for the past several weeks about his belief that the country "will never truly heal until we address the original sin of slavery."

"I've said that if under our Constitution that we compensate if take their property, why wouldn't we compensate people who were considered property and sanctioned by the state?" Castro said during his appearance at the NAN conference Wednesday. "I believe that for the black community, for the white community, for every other American -- that it is important to address that original sin, and until we do we may feel like we're moving forward as one nation, but I don't think that we ever will."

Former Starbucks Chairman and CEO Howard Schultz, who is exploring an independent run for the White House, said he would not support reparations during a Fox Town Hall on Thursday night, adding that he would "rather look forward" and was open to other ideas like making a significant investment in education that could begin with historically black colleges. He was not asked about the Jackson Lee legislation.