They issued a subpoena to Mulvaney, who refused to comply. Clearly, Executive Privilege could be used as the reason. But Trump's attorneys argue that the subpoena was invalid because the underlying reason for issuing it was the "impeachment inquiry", but the problem with that is Nazi Pelousy never held the required vote to implement an impeachment inquiry. The reason for refusing to comply doesn't matter. Normal protocol when a subpoena is denied is to take it to the courts to get a ruling. Why didn't Nazi and her House Clowns follow normal protocol? The withdrew the Muclvaney subpeona and did not go to court. Instead, they rushed a vote and are now over at the Senate begging jurors to conduct an investigation that was their responsibility. Board Dimwingers refuse to acknowledge the responsibility for not having the witnesses they now claim is crucial to their "overwhelming" case is on the House Clowns. Not Trump. Not the Senate. Once again, don't argue whether or not the reason for challenging the subpoena is valid. That will get nothing done here. That's is now a question for the courts. Dimwingers refused to go to the courts. So stop trying to blame Trump and the Senate for the failures of your House Clowns to do their job.