General Discussion

It seems to go back to the McCarthy era:

Throughout this time, McCarthy — in an eerie prescience suggesting today’s conspiracy theories about Democrats drinking children’s blood or Haitian immigrants eating pets — argued that the central nexus of the “homosexual agenda” and the “communist sympathizers” was the “Democrat Party.”
Throughout his hearings, carried on live television every day for almost two months, McCarthy refused to use the actual name of the party and its members he was slandering.

Instead, he argued, “Democratic” sounded too nice, too, well, democratic. McCarthy is sometimes quoted (it may be apocryphal) as saying, in Newt Gingrich Word List fashion, that Republicans should instead always say, “Democrat Party, with the emphasis on the ‘rat’!”

In fact, the slur has persisted. Mitch McConnell refers to the “Democrat Party” from the well of the Senate. Random Republicans repeat the slur on television more frequently than they use the actual name of the party. Conservative media use it almost exclusively to describe the Democratic Party.
https://hartmannreport.com/p/weaponizing-words-the-gops-plot-to-61c