Since I can't find my original source anymore, I cannot defend this statement right now - consider it retracted. There are multiple sources for "majority democrat", but nothing close to 90%.
What I did find, and what I possibly confused it with, was that political spending of the respective government employee unions are overwhelmingly democrat:
"Of course, it’s not just teachers’ unions. In the 2020 election cycle alone, the American Federation of State, County and Municipal Employees (AFSCME) dedicated 99.1% of its political spending to Democrats. The American Federation of Government Employees gave 95.6% to Democrats." [0]
Yeah gonna need a source for that.