While State Representative, I attended conventions of the American Legislative Exchange Council. ALEC, for short.
In an informal conversation with a Mississippi State Rep., I was told that the law that allowed only the top two finishers in a primary election to advance to the general election was a scheme to prevent blacks from winning office.
That law was instituted in California to keep Republicans off general election ballots.
The result was no Republican being on the ballot there for U.S. Senate last year.
Now, according to the Wall Street Journal, Democrats are advancing the idea elsewhere:
Harvesting Democratic Votes Liberals want to impose the California voting model on all 50 states. – Editorial