Nixon was never impeached but he left.
Nixon's approval ratings were markedly higher than trumps are when the impeachment inquiry started. As they got into the pre-impeachment process his pole numbers collapsed.
So history tells us we don't need an actual impeachment vote. We need the inquiry. And while trump may never drop below 30%, the rest of his Republican team WILL drop, and they will drop precipitously.
And when that happens, either they will impeach, AND CONVICT, or they will lose mightily in 2020.
My choice is that they lose mightily in 2020. Maybe if we got 64 senate seats, we could impeach Kavanaugh. Heaven knows the malfeasance is there.
And maybe we can also impeach half the Federal judges Trump has appointed. Again, I suspect the malfeasance is there for surely that many.