Mark Levin dug this up on Senator Leaky Leahy reference executive subpoenas:

Excerpt from Statement of Senator Patrick Leahy on September 23, 1999:

“Most importantly, compelling compliance with a congressional subpoena in this context would be difficult. The civil contempt mechanism normally available to Congress, see 28 U.S.C. ¬ß 1365, specifically exempts subpoenas to the executive branch. The criminal contempt mechanism, see 2 U.S.C. ¬ß 192, which punishes as a misdemeanor a refusal to testify or produce documents to Congress, requires a referral to the Justice Department, which is not likely to pursue compliance in the likely event that the President asserts executive privilege in response to the request for certain documents or testimony. Thus, the only legal way to enforce this subpoena would be to hold a witness in contempt using its ‚Äúinherent contempt authority,‚Äù but this would require a contempt trial on the floor of the Senate. Not many of us relished our role as jurors during the impeachment trial and are not anxious to reprise that role.”

UPDATE: Much more at Patterico.