So you go along with their definition of "extremism?"
(U) Rightwing extremism in the United States can be broadly divided into those groups, movements, and adherents that are primarily hate-oriented (based on hatred of particular religious, racial or ethnic groups), and those that are mainly antigovernment, rejecting federal authority in favor of state or local authority, or rejecting government authority entirely. It may include groups and individuals that are dedicated to a single issue, such as opposition to abortion or immigration.
A bit more inclusive than it used to be, isn't it? It's interesting that the Tenth Amendment is now apparently subversive, as is holding the "wrong" opinions on abortion or immigration.