So these formerly registered democrats registered republican to vote in the primary, even though they have been voting republican in the general elections for years?
Edit - hopefully this post isn't coming across as snark (it kind of reads that way to me now). I'm genuinely asking as I want to make sure I understand those tweets.
yes, that's exactly it.
The parties actually used to be in opposite positions. the republican party was the progressive, abolitionist movement, the democratic party was full of racist southerners who would NEVER EVER vote republican because of lincoln and the civil war.
The realignment around the civil rights act of 1964 changed this. northern democrats and republicans largely backed that bill, southern democrats and republicans opposed it. (this is an oversimplification, but go with it).
"southern democrat" politicians split briefly into their own (extremely racist) party, then eventually joined with southern republicans to make a "new" highly racist republican party.
This all took place at the elected official level, mostly. It took the rank and file a little while longer to catch on- so there were for decades after that STILL a lot of southern "democrats" who were still registered as such, but voted reliably republican every year. Since most people don't vote in primaries, there's no real need to go through the trouble to change that registration- unless there is a competitive primary race which doesn't happen all that often.