There are three key precedents that prohibit the government from banning semiautomatic firearms.
U.S. v. Miller - Protected arms are those "in common use" that "bear some reasonable relationship to the preservation . . . of a well-regulated militia".
D.C. v. Heller - "The Second Amendment protects an individual right to possess a firearm unconnected with service in a militia, and to use that arm for traditionally lawful purposes[.]
McDonald v. City of Chicago - "We therefore hold that the Due Process Clause of the Fourteenth Amendment incorporates the Second Amendment right recognized in Heller.”
These are not mere inconveniences for the Democrats’ argument, they are fatal to it.