From Presidential Advisers’ Testimony Before Congressional Committees: An Overview (Dec. 15, 2014 RL31351):
Congress has a constitutionally rooted right of access to the information it needs to perform its Article I legislative and oversight functions. Generally, a congressional committee with jurisdiction over the subject matter, which is conducting an authorized investigation for legislative or oversight purposes, has a right to information held by the executive branch in the absence of either a valid claim of constitutional privilege by the executive or a statutory provision whereby Congress has limited its constitutional right to information.
A congressional committee may request (informally or by a letter from the committee chair, perhaps co-signed by the ranking Member) or demand (pursuant to subpoena) the testimony of a presidential adviser. However, Congress may encounter legal and political problems in attempting to enforce a subpoena to a presidential adviser. Conflicts concerning congressional requests or demands for executive branch testimony or documents often involve extensive negotiations and may be resolved by some form of compromise as to, inter alia, the scope of the testimony or information to be provided to Congress.