From Article IV, section 4 of the US Constitution.
The United States Shall Guarantee to every state in the union a republican form of government and shall protect each of them against invasion; and on Application for the Legislature, or the Executive (when the Legislature cannot be convened) against domestic Violence.
Yeah. Tell that to Arizona. They’ve been invaded. Their reward for trying to do something about it was a lawsuit from the federal government to protect the invaders.