There is one thing missing in 2018 in comparison to 1974: Media Credibility. Americans so distrust the media they will discount a lot of evidence that would have convicted a conservative in a previous era. I think Trump is fairly safe from impeachment.
If anything, putting the issue front and center before the midterms may well help the GOP. Moderate voters do not want to see a constitutional crisis, but Democrat candidates cannot afford to alienate the wackos out there calling for Trump's head.