If POTUS breaks the law {felony level} or deliberately breaches national security such as unauthorized release of classified material, POTUS should like any other citizen be subject to arrest and prosecution. The same agency which oversees POTUS security should also be bound by law to report any known violations by POTUS as well as arrest for same. Yea I know I'm dreaming. Not even POTUS should be above the law.
POTUS has presidential immunity.
This may be unfortunate, and lead to bad results, but it is the law.