COLUMBIA, S.C. (AP) — Democratic presidential contenders are celebrating the Martin Luther King Jr. holiday and his legacy in South Carolina, Iowa and elsewhere. In Columbia, the field’s major contenders linked arms with each other and civil rights leaders in a crowd of thousands marching to the South Carolina Statehouse. Bernie Sanders and Elizabeth Warren put aside their frosty relationship of recent days and shook hands at a church service joined by other rivals, too. South Carolina’s first-in-the-South primary is a crucial proving ground for a candidate’s mettle with black voters. The state’s showcase holiday celebration, Columbia’s King Day at the Dome, is a notable event for Democratic politicians.