Not for a riot, he can't. Read the Constitution! The Pres can call out the militia for a war or rebellion, and then the federal government must pay them and treat them like regular troops. Within a state he can't even declare martial law -- only the Governor can do that, and then the NG would go in first. Examine the riots of the 1960s -- Watts, Chicago, Detroit. Federal troops were used ONLY after the request of the Governor and aas back-up for the NG. I know; I was there.