White primary: A Democratic party primary in the old "one-party South" that was limited to white people and essentially constituted an election, ruled unconstitutional in Smith v. Allwright (1944). Here's the PBS website on Jim Crow and white primaries.
Sentence
After Reconstruction, since the vast majority of southern voters were Democrats Republicans could never beat democrats in a general election. Blacks were allowed to vote in general elections, which didn't actually decide anything, but not in the primary, the only vote that actually mattered.