I was a senior in college when the Watergate investigation was really heating up, so I recall it reasonably well. The pressure placed on Nixon to resign mounted by the day in 1974. He denied the possibility of conviction in the Senate (as I recall, at least privately he was resigned to the House voting out one or more articles of impeachment and taking his chances that there would never be a 2/3 vote to convict in the Senate). That changed when senators Hugh Scott (R, PA) and Barry Goldwater (R, AZ) paid him a visit and told him flatly that he didn't have the votes in the Senate to avoid conviction. Then it became a choice of resignation on more or less his own terms and going down in flames. Thus, the announcement and subsequent resignation.
While the subject of the '76 election is out there: that one was bitterly close all the way, and it would have taken only some very minor twitches to invert the result. And when I say that, I mean the networks didn't make a call until about 5:30 AM on Wednesday morning following the election (on ABC, Harry Reasoner looked like he'd been dragged through a knothole, and Barbara Walters looked like she'd aged 15 years during the course of the evening). For a benchmark, consider that the networks called the election of 1980 at about 8:30 PM eastern standard time on the evening of the day of the election.