Biden’s AG Merrick Garland is in contempt of Congress right now - when does his prison sentence start?
Republicans who ignore a subpoena from a democrat committee and a democrat runs the DOJ, they will be prosecuted. And since the crime occurred in D.C., they will convicted.
A democrat who ignores a subpoena from a Republican committee and a democrat runs the DOJ, there will be no prosecution.
So to answer your question, Garland will to decline to prosecute himself, just as Eric Holder declined to prosecute himself.
It’s all really simple really.