Unfortunately it’s a real thing among leftists (not necessarily Democrats in general).

The belief is that any advanced classes increase the achievement gap. People who subscribe to this also believe that advanced placement testing is discrimination and must be eliminated. They want equity of outcome, so reducing the curriculum to a single class at a single level that everyone the same age takes is their preference.

It has been implemented in several places with predictable backlash.

The funny, and somewhat ironic thing is from my own understanding of Communist nations is they don't even try to do this... USSR put in a fair amount of effort to promote excellence and achievement for students into more specialized programs earlier than later. With an overall increased divide between the best and worst performing. I find it hard to fathom that there are those in the US that work so hard to hold back advancement from those most capable.