The Republican Party itself has been infiltrated by “hydra” [symbolically if not actually — this is a reference to a Captain America movie]. Thus, I believe the Republican Party itself is Replacement Theory in reality [over time]. It certainly is no longer conservative or constitutional. The Republican Party throughout the USA is using lies, misrepresentation, threats, and intimidation to create replacement of peoples normal behavior and concepts and even trying to replace normal votes with only their desired votes.
Also, America would not exist except we replaced the American Indians on their own territories [through violence, in most cases].
Russia is currently using replacement and exportation of Ukraine citizens to take territory from the Ukraine.
Even, the Catholic Church used replacement [theology through violence] in Middle and South America as well as other World locations.
Basically, I don’t see that the Democrats are using immigrants in a replacement theory situation. I don’t see anyone using replacement theory except for Republicans, extremists, and other such groups.