COLUMBIA, S.C. — As Americans commemorate Martin Luther King Jr.’s contributions to the nation, Democratic presidential hopefuls are fanning out across the country to honor the civil rights leader. Sen. Kamala Harris, D-Calif., used the holiday to launch a presidential campaign that, if successful, would make her the first woman and the second black candidate to become president. Meanwhile, an annual rally to observe King’s birthday held in the capital of South Carolina, a critical early-voting state in the Democratic primary, will feature two senators expected to seek the White House in 2020, Cory Booker of New Jersey and Bernie Sanders of Vermont. Former Vice President Joe Biden, who’s weighing his own presidential bid, is set to speak at a King holiday event in Washington alongside former New York mayor and possible 2020 rival Michael Bloomberg.