In 1797, John Adams came into possession of an incriminating letter implicating Tennessee Sen. William Blount in a half-cocked land speculation scheme to conspire with the British to seize Spanish Florida. Adams sent the letter to both the House and Senate along with an opinion from the attorney general that Blount had committed a crime and that members of Congress were subject to impeachment.
The House promptly impeached him. The Senate also voted to expel him. The impeachment was taken up, but the Senate eventually voted down a resolution asserting that Blount was an impeachable officer. It wasn’t specified if this result was because members of Congress weren’t impeachable, or because Blount had left office when he was expelled. But it became the accepted interpretation that members of Congress are only subject to expulsion by their respective chambers, they are not “civil officers” who can be impeached. No attempt has been made since to impeach, rather than expel, a member of Congress.
So, Mitt Romney is not the first Senator to be be target of a presidential impeachment recommendation. Technically not unprecedented.