By Trevor Hunnicutt and Nandita Bose COLUMBIA, South Carolina (Reuters) -U.S. Vice President Kamala Harris warned Americans that their freedom is under threat as she commemorated Martin Luther King Jr. Day in early-voting South Carolina on Monday, wielding the civil-rights icon’s legacy to urge Black voters to join Democrats to win the 2024 election. Harris headlined an annual event by the NAACP, the nation’s oldest civil rights group, which included a prayer service and a march to the South Carolina House of Representatives in Columbia. She pressed one of Democrats’ central election messages …