Nixon quit. No articles of impeachment, though there were actual laws he broke that could have been used against him. Clinton committed perjury. That is an impeachable offense. There was no "could have" committed perjury. He did it and everyone knew it. In the end, it was decided that his perjury wasn't to the level that should throw him out of office. It was the right choice to make.
But your post says this................
"Could be impeachable" is NOT grounds enough to bring on impeachment. For both Nixon and Clinton they had more than just a "could be".
That's what makes this impeachment nothing but silliness, stupidity, and vacant wishes for Democrats. It will hurt them more than it helps them.