I think that the Senate could also ask for an investigation to see whether the President broke the law. Actually, I think that the House of Representatives can call for an investigation as well. But after that, I'm not sure --

The best parallel would be Watergate, though -- President breaks law, big investigation, impeachment imminent, pres steps down.

So there should be legal precedent in Watergate. I just can't remember how it happened, exactly. I think it was a Senate investigation.