There is plenty of precedent for a president to send Federal troops into a state, or to just Federalize the state's NG, to see that Federal law is upheld when the states refuse to do so. See Orval Faubus and DD Eisenhower in Little Rock, Arkansas, and George Wallace and LBJohnson in Alabama.
When it comes down to the nut-cutting, though, the Federal government already has the Constitutional authority to come into a state and uphold/enforce Federal law.
Remember 1861-1865. The Federal Government proved then that it is supreme and in the 1950 and 1960 it proved it again. The only thing different now is that the Federal Government has more power. Larry