Back in 1868, President Andrew Johnson was impeached for violating the Tenure of Office Act of 1867 when he tried to remove the secretary of war Edwin Stanton without the Senate's consent. The specific portion that the Senate used in the main article of implachment was Section 2 of the act:
But if the Senate shall refuse to concur in such suspension, such officer so suspended shall forthwith resume the functions of his office, and the powers of the person so performing its duties in his stead shall cease, and the official salary and emoluments of such officer shall, during such suspension, belong to the person so performing the duties thereof, and no to the officer so suspended.
Even though Andrew Johnson's impeachment failed, it is likely that this act would have been rendered unconstitutional as per Myers v. United States:
When on the merits we find our conclusion strongly favoring the view which prevailed in the First Congress, we have no hesitation in holding that conclusion to be correct; and it therefore follows that the Tenure of Office Act of 1867, in so far as it attempted to prevent the President from removing executive officers who had been appointed by him by and with the advice and consent of the Senate, was invalid, and that subsequent legislation of the same effect was equally so.
This sparked my curiosity around articles of impeachment, namely...
If a law used for an article of impeachment is rendered unconstitutional, is that article of impeachment nullified?