Congress is empowered to impeach and remove from office cabinet officers, though it has only impeached one of them, president Grant’s Secretary of War, William W. Belknap. He resigned on the same day as his impeachment, so was not convicted by the senate.
If the Republicans regain control of the senate, they would still have to persuade enough Democrat senators to vote to have the 2/3rds majority to remove Holder from office.
If Holder can't produce a warrant for those AP wiretaps, he is toast.