Mississippi Democrats are deciding the last in a series of presidential contests between Barack Obama and Hillary Clinton.
Voting appeared heaviest in areas offering both presidential and congressional primaries.
In some areas, circuit clerks' offices say morning rain may have slowed early voters.
Clinton campaigned in Mississippi last week, but by Monday was in Pennsylvania, where the primary is April 22.
Obama, making a final stop Tuesday morning in Greenville before flying to Pennsylvania, noted that the Mississippi Delta area is struggling economically.
He told a small gathering at Buck's Restaurant that he has not had as much opportunity as he would to come to the area.