1) If they do this, they will give rise to a definite 100% certainty of President Trump’s reelection.
2) If they do this, they may even give rise to acts of violence directed at them.
3) If they do this, they will start a new tradition, that every President from now on will be impeached by the opposing party.
If the Dims impeach, then they stay focused on 2016, consequently do nothing to prepare for 2020, and it all blows up in their faces and we win bigly. If they don't impeach, then Trump calls them out as the worthless cowards that they are, it blows up in their faces, and we win bigly in 2020.
They can choose their poison.