No there is no single entity that does what you describe. However there are several institutions that can curb Presidential authority.
Executive Branch Curbs
Executive branch members swear an oath to defend the Constitution that pre-empts all other obligations. The President cannot expect an unlawful order to be carried out.
The White House Counsel and Department of Justice serve as sounding boards to ensure executive actions initiated by the President are lawful prior to implementation, although sometimes they get it wrong.
Almost all executive branch organizations have an Inspector General that oversees abuses within their purview.
Checks and Balances with the Congress and Judiciary
United States Department of Justice Office of Special Counsel can be called to prosecute specific acts by Congress and the Attorney General.
The Congress can investigate through it's committee structure and subpoena power Executive Branch activities and acts, and ultimately impeach and remove the president from power.
The Congress also has the Congressional Research Service and the Library of Congress (and its Law Library) which can conduct analysis and legal research independently.
The Courts and especially the Supreme Court can overturn laws and acts that are determined unconstitutional.
The people have the ability to remove him from office during election.