If local authorities cannot protect the citizens the president is within his right to use active duty troops, or if the insurrection act is enforced. Bush did the same thing in 1992. The president can also act unilaterally to protect federal buildings or functions. Nothing about what Trump is doing is illegal.
Okay so just ignore what I said? MAYOR of DC BOWSER deliberately stopped attempts to call in the National Guard, and speaker of the House is in charge for establishing police presence. Nancy Pelosi’s daughter even has her ON CAMERA saying “I accept responsibility for January 6th”