I remember Watergate. In 1972 the conventional wisdom about impeachment was perhaps captured in JFK's Profiles in Courage--the impeachment of Andrew Johnson was wrong, very wrong, and the country was only saved by a Kansas senator's courage (IIRC--not bothering to look it up). The country had skated up to the edge then but had wisely drawn back. Impeachment was a constitutional dead letter, almost on a par with stationing soldiers in homes (Third Amendment), possibly used in the odd case of a judge, but not for presidents.
As Watergate unraveled, impeachment started to become possible. Then in the summer of 1974 suddenly things clicked into place and the avalanche started.
Will history repeat itself?
I don't think so--Republican support of Trump seems too solid, but as Watergate shows surprises can happen.