I am of the opinion that the Dems will never push for impeachment over the Russian Collusion charge as they know that they are the source of the collusion and don't want the back lash.
For Congress critters in purple districts, a vote for impeachment will be equivalent to a letter of resignation. It's one thing to sit in the back and let the bomb throwers shout from the rafters, it's entirely another thing to have your vote recorded for the ages. There will be no dancing around a vote for impeachment.
First, the Democrats will have to come up with specific articles of impeachment. From my perch, presumably as a hated "Non-Trumper", I see nothing done by Trump that meets any objective standard.
I just don't see how promising impeachment will put the Democrats over the top in November.