“The same person who denied their request is the person who Congress would have to ask to enforce the subpoena...”
Not true.
Congress has unquestioned authority to jail anyone in defiance of a Congressional subpoena. The idea that they have to ask the DOJ to prosecute is a fallacious myth. The Speaker Of The House can direct the Sgt. at Arms to arrest and incarcerate the subject of the subpoena until compliance is obtained.
L
True, they could do that, but when is the last time Congress has actually done it?
Even if they did arrest someone at the DOJ for contempt, they would still be able to claim executive privilege in a court and challenge the subpoena and arrest. So this power, when it comes to be used against the Executive branch, certainly isn’t guaranteed or even reliable.