One of the most important days in the Democratic Party’s presidential contest arrives today. It involves neither a caucus nor a primary, not a kickoff nor concession speech, and no direct confrontations—party leaders hope—between the candidates. Indeed, for many Americans, it’s a holiday.

It’s Martin Luther King Jr. Day, and how it goes will tell us much about the racial and religious politics in the Democratic contest.

Harnessing Dr. King’s memory has always been a political prize, but it is especially valuable now with the next Democratic primary slated for South Carolina. The stakes could not be higher: Black voters are expected to make up as much as 50 percent of the Democratic electorate in the Palmetto State, and large numbers are devoutly religious.

Democratic front-runners Barack Obama and Hillary Clinton have worked hard to appeal to these voters. Obama speaks of his time as a community organizer working with churches in some of Chicago’s poorest neighborhoods, and his speaking style has been compared to King’s. Meanwhile, Clinton has long been popular among African Americans, in part due to her public advocacy work and in part due to her husband, whom Toni Morrison once called America’s “first black president.”

Clinton and Obama both want to grasp King’s mantle—because they believe in his ideals and because they know his legacy brings sizeable capital. That goes far to explain the tit-for-tat dustup between the campaigns last week over King’s legacy.

It’s not clear what exactly Clinton meant when she said, in New Hampshire shortly before that state’s Jan. 8 primary, “Dr. King’s dream began to be realized when President [Lyndon] Johnson passed the Civil Rights Act. It took a president to get it done.” That Clinton sought to minimize King’s importance seems unlikely, but at a minimum the words legitimately raised eyebrows.

What we do know is that in days afterward, when Obama and Clinton traded criticisms and accusations through news media and campaign surrogates, the Democratic Party’s worst nightmare was at hand—a brewing divide over race when the White House is so visibly in sight. Obama and Clinton reached a rhetorical cease-fire last week, but this will be tested by the symbolic weight of King’s holiday. Further, both candidates are aware that in the Nevada caucuses on Saturday, exit polls showed that 8 of every 10 African Americans voted for Obama.

But King’s legacy is not only about race. King also was a religious leader whom many Americans—particularly on the political left—view as a model of how faith and politics should connect. And again, the stakes could hardly be higher. Our analysis of more than 15,000 public communications by political leaders from Franklin Roosevelt’s election in 1932—the beginning of the modern presidency—through six years of George W. Bush’s administration shows that since the political rise of religious conservatives in the late 1970s every successful presidential candidate has substantially emphasized religious faith. It’s a point the Democrats learned anew in 2004.

Obama laid a religious groundwork for his campaign early, including a decision to make South Carolina the focus of his campaign’s “40 days of Faith and Family” last autumn. He has been a regular speaker in churches, particularly those with predominantly African American congregations, and typically slips into his most King-like idiom in these contexts. Obama often begins speeches by giving “all praise and honor to God,” a phrase familiar in the black church.

Clinton too has made faith central to her campaign. In November she received a standing ovation after her speech at a Global Summit on AIDS and the Church hosted by Rick Warren, perhaps the most influential evangelical in the world. In South Carolina on Sunday, Clinton visited Columbia’s Northminster Presbyterian Church, where she worshipped, spoke, and even sang with the choir.

All of this means that Martin Luther King Jr. Day offers a potent moment for Obama and Clinton to embrace faith and emphasize racial equality with public, high-profile events. That two leading candidates for the presidency are doing so is one measure of King’s legacy. That the candidates are a white woman and black man is perhaps an even more remarkable one.

These points are what the candidates should emphasize today—and all of this week in South Carolina—if they want to unite the Democratic Party and grasp King’s mantle.