"Obstructive act The President's repeated efforts to get McGahn to create a record denying that the President had directed him to remove the Special Counsel would qualify as an obstructive act if it had the natural tendency to constrain McGahn from testifying truthfully or to undermine his credibility as a potential witness if he testified consistently with his memory... The President's assertion in the Oval Office meeting that he had never directed McGahn to have the Special Counsel removed... runs counter to the evidence."