Black Lives Matter protesters chant during a Hillary Clinton rally in Atlanta last month. (Melina Mara/The Washington Post)

Criminal justice pushed its way into the forefront of the 2016 presidential conversation this summer when activists disrupted candidates on the campaign trail, demanding that they acknowledge the disproportionate impact of law enforcement on black communities.

Yet only three of the 17 Republicans and Democrats running for president have agreed to participate in the first national candidates forum focused on criminal justice, sponsored by a bipartisan group of black political, business and civic leaders. The event takes place Saturday at Allen University, a historically black college in Columbia, S.C.

Neither Hillary Clinton nor Donald Trump, the two major party front-runners, is scheduled to appear. And of the 14 candidates vying for the Republican nomination, only Ben Carson has agreed to show up to discuss what is a major concern for many black voters. The cable network Black Entertainment Television, a co-sponsor, will stream the event live.

The sparse participation by candidates underscores the question of whether the issue has gotten the full attention of the 2016 field despite several highly publicized incidents, including one last month in Atlanta when activists chanted for 15 minutes while Clinton described her plans for criminal justice reform.

Instead, the debate that has roiled the country for three years has often been reduced by some to whether candidates are willing to utter the three words that have come to symbolize the movement: “Black lives matter.”

Social video shows a group of Black Lives Matter protestors disrupted a Hillary Clinton rally at Clark Atlanta University, a historically black college in Atlanta. (Elizabeth Montgomery/Instagram)

Democrats have generally come to embrace the slogan, while Republicans have avoided it — some calling it racially divisive and insisting instead that “all lives matter.” Some GOP candidates — notably Sen. Ted Cruz (Tex.) and New Jersey Gov. Chris Christie — have called the movement and the politicians who support it hostile to law enforcement officers.

Other groups affiliated with the Black Lives Matter movement are working with the national political parties to hold town halls and party debates focused on criminal justice.

“It’s a missed opportunity that the top candidates for both parties will not be here to discuss in depth the issues we think are paramount in this election cycle for black Americans,” said Ashley D. Bell, a co-founder of 20/20 Leaders of America.

The group takes its name from the makeup of its leadership: 20 Democrats and 20 Republicans who have agreed to work together to push candidates for the presidency to be accountable to the black community.

But Carson, who is African American and whose personal story of rising from poverty to become a renowned neurosurgeon has resonated with many black people, is the only Republican candidate who agreed to participate in the forum. Former Florida governor Jeb Bush is sending a surrogate, Kay Coles James, who was director of the Office of Personnel Management during the administration of his brother, former president George W. Bush.

Trump, who declined to participate in the event, is set to speak Friday at the Commander-in-Chief Presidential Forum series in Spartanburg, S.C., about 90 miles from Columbia. Sen. Rand Paul (Ky.), who before announcing his campaign earlier this year often went before black audiences to talk about criminal justice reform, is not attending.

On the Democratic side, Clinton will be campaigning in Charleston on Saturday and will be represented at the forum by Mayor Sly James of Kansas City. Her two rivals for the Democratic nomination, Sen. Bernie Sanders (Vt.) and former Maryland governor Martin O’Malley, will be on the stage in Columbia.

Bell, a former president of the College Democrats of America who switched to the GOP in 2010, said the 20/20 group and the criminal justice forum are efforts to “make sure that one party doesn’t take us for granted and the other sees us as a vote that can and should be fought over.”

The three contenders for the Democratic nomination, who have been directly challenged by Black Lives Matter activists at campaign events, have responded by promising to pursue policies that address concerns of the movement, which grew out of protests over the slayings of unarmed black men by law enforcement officers or gun-wielding civilians.

One of Clinton’s first major speeches after launching her campaign was to declare the “need to restore balance to our criminal justice system” and call for “an end to the era of mass incarceration.” In recent weeks, she has fleshed out her plans, which include legislation to ban racial profiling and to prohibit the federal government and its contractors from asking job candidates about their criminal records in the initial stages of the application process.

During the summer, Sanders and O’Malley came out with more-detailed plans for criminal justice reform. O’Malley’s plan calls for addressing the disproportionate rate of school suspensions and expulsions, as well as arrests and detention, of children of color. Sanders’s proposals, which he has framed as “racial justice,” include tackling the high rate of unemployment and poverty in communities of color.

No mention of criminal justice reform was found in the issues sections of campaign Web sites for top Republican contenders, including Trump, Carson, Bush and Sen. Marco Rubio (Fla.). Rubio said in an interview several months ago that the issue has legitimacy, citing black friends who have recounted being frequently stopped by police for no apparent cause.

During a campaign rally for Bush in Las Vegas, activists began chanting “Black lives matter” after being dissatisfied with how he answered a question about racial inequality.

One of the founders of Black Lives Matter, Patrisse Cullors, said candidates need to do more than just say the words.

“It’s extremely narrow and extremely dismissive when candidates sort of blurt out ‘Black lives matter’ and don’t give any real substance to what that means,” Cullors said.

She and other activists are working on a platform that “really does calculate the ways in which black people have been impacted in all our lives, not just police killings,” she said. She cited the ongoing campaign by fast-food workers for a $15-an-hour minimum wage and black domestic workers’ fight for recognition. “There is a larger conversation to be had.”

In Gallup polling during the past three months, black respondents have cited unemployment and the economy as issues that are as important as race relations, according to pollster Jeff Jones. He also noted that race relations “are of much greater concern” for blacks than for whites.

Candidates were asked one question about racism or the Black Lives Matter movement in each of the two Democratic debates, and on the Republican side, the subjects came up only once, in their first debate. Cullors is among a group of activists still pushing the Democratic National Committee to host a debate focused on issues affecting black communities.

The DNC has said it will not add a debate, but it has welcomed the idea of a town hall. Cullors said that given several Republican candidates’ reaction to the movement, she is even less hopeful about the chances of the Republican National Committee agreeing to a debate.

“I think it’s great that a town hall is happening that focuses on criminal justice reform. We’re not against town halls. We just don’t think they are a substitute for a debate,” Cullors said. “We don’t believe the DNC should get away with not listening to the community’s needs, and we want to challenge them in this moment.”