The qualifier here is "could". Meaning they are not there yet, but are well on their way there.
I'm going to go with Hitler's Nazi Party and Stalin's Communist Party instead. They killed about 21M and 62M respectively. The Republicans are off to an awfully slow start.
If they want to kill many, they first need to make it a requirement that there be only one party and require people to be members or else suffer extreme consequences. Then it is easy for them to commit atrocities and commit mas murder.