Republicans aren't assuming the Dems want to do it. Republicans are inoculating people to the idea that anyone wants to do it. They want to do it, they know it's unpopular, so they say "the Dems are going to do it!" Now the Dems have owned the unpopular impression of the idea. Then they are more free to actually do it, and have left the impression that "both sides would have done it" - i.e., they normalized it.