There were those daily recordings in the oval office revealed by Butterfield. Then John Dean spilled the beans because he figured out he was being set up as the fall guy. Those recordings indicated strongly that Nixon ordered the cover up. The WH was checking daily on how Nixon supporters would vote, and when evidence pointed to probable cause, supporters had to switch sides, and it was an honest reckoning, and thus lose in the House. That's when he resigned, and some believe his reasoning was to save the country.