Southern black voters don’t usually play a decisive role in national elections. They were systematically disenfranchised for 100 years after the end of the Civil War. Since the days of Jim Crow, a fairly unified white Southern vote has often determined the outcome of elections.

This November could be different. Nearly five decades after the passage of the Voting Rights Act, black voters in the South are poised to play a pivotal role in this year’s midterm elections. If Democrats win the South and hold the Senate, they will do so because of Southern black voters.

The timing — 50 years after the passage of the Civil Rights Act and 49 years after the passage of the Voting Rights Act — is not entirely coincidental. The trends increasing the clout of black voters reflect a complete cycle of generational replacement in the post-Jim Crow era. White voters who came of age as loyal Democrats have largely died off, while the vast majority of black voters have been able to vote for their entire adult lives — and many have developed the habit of doing so.

This year’s closest contests include North Carolina, Louisiana and Georgia. Black voters will most likely represent more than half of all Democratic voters in Louisiana and Georgia, and nearly half in North Carolina. Arkansas, another state with a large black population, is also among the competitive states.