Congress has given the President the power to take control of the D.C. local police for up to 30 days "for federal purposes," and to deploy the (tiny) D.C. National Guard.
As today's "One First" explains, that's the full extent of what the President can do to "federalize" D.C. without new statutes:
172. "Federalizing" D.C.
Like any federal enclave, the federal government has plenary power over the District of Columbia. But Congress has delegated most of that power to local officials; it would take new laws to undo that.