The House and Senate can vote a statutory contempt citation if the conditions of a subpoena are not met. By law, the contempt citation is then referred to the US Attorney for the District of Columbia "whose duty it shall be to bring the matter before the grand jury for its action." With regard to citations in response to claims of Executive privilege in the US Attorney firings scandal, unnamed Administration officials have asserted that the Congress has no power to force the Department of Justice through the US Attorney for DC to convene a grand jury and pursue charges of contempt. They are basing this position on an untested May 30, 1984 opinion by then head of the Office of Legal Counsel (OLC) Theodore Olson. Olson argued that since the Executive executes the laws in specific cases, it is an infringement of the separation of powers that Congress should substitute its judgment for the Executive’s and direct it to apply a law against any specific individual. He then seeks to marry this concept to the situation of an individual receiving a contempt citation from the Congress in an Executive privilege case. According to Olson, prosecutorial discretion allows the Executive to decline to pursue such a case.
What Olson is really arguing, however, is not separation of powers but that the Executive is an independent power.
"The Executive’s exclusive authority to prosecute violations of the law gives rise to the corollary that neither the Judicial nor the Legislative Branches may directly interfere with the prosecutorial discretion of the Executive by directing the Executive Branch to prosecute particular individuals." p.115
As such, the Executive is unaccountable to anyone in how the laws are executed, except in the broadest terms I suppose of election, funding, and impeachment. Olson’s argument also assumes the duties of US Attorneys are identical with the interests of the Executive, which in practice are those of the sitting President. If this view were to be accepted, US Attorneys would cease to be agents of the law and become agents of a particular President and his/her agenda. In other words, they would become political not legal officers. It was precisely the Administration’s attempts to erase this difference which produced the Attorney firings scandal in the first place.