[Since the United States, under our Constitution, cannot adopt Sharia Law, why isn't Islamism illegal in the US since it suggests the overthrow of the Constitution?]
What is Islamism?
Islamism is an ideology which holds that political action within society should be founded on the doctrine of Islam. Its main objective is the creation of an Islamic state rooted in the sharia (Muslim law) to transform all aspects of society, such as economy, law, social justice, and foreign policy.
Islamism is a highly heterogeneous movement encompassing dozens of Islamist political groups with contrasting agendas, values, and affiliations to the different branches of Islam. It comprises democratic, conservative, and moderate groups as well as radicalized associations and terrorist organizations.
Despite their differences, all Islamist movements seek to transform society through Islamic education and political and social action. Most of them hold that power should be attained by democratic means. They advocate a return to "authentic Islam", as opposed to the Westernization of society that is taking place as a result of globalization.
https://humanidades.com/en/islamism/