"As you know, there have been only three prior instances in our nation's history when the full House has moved to formally investigate whether sufficient grounds exist for the impeachment of a sitting President," writes McCarthy. "I should hope that if such an extraordinary step were to be contemplated a fourth time it would be conducted with an eye towards fairness, objectivity and impartiality." 

"Unfortunately, you have given no clear indication as to how your impeachment inquiry will proceed - including whether key historical precedents or basic standards of due process will be observed."