Impeachment is a political process, and it becomes much more likely when Trump is extremely unpopular . Obviously a significant criminal investigation(s) could affect his popularity, but we haven’t seen hard evidence for collusion (and presumably his voting base will be looking for a higher standard of evidence than your average liberal voter – or maybe they won’t care either way). The state of the economy may play an even bigger role in his popularity, but I doubt big macroeconomic effects from crappy Republican policies will hit us in the two year timeframe, but maybe in four years?
Presuming Trump’s popularity plummets and he is shunned by some in his own party, impeachment is possible with a Republican House (simple majority vote) but I doubt he would be removed from office in the Senate (2/3 majority). I don’t believe many Republican legislators have principles but they do seem to look out for their own ilk. Obviously the likelihood of impeachment could increase in 2018 if the balance of power shifts to Democratic control. And what does the balance of power look like after 2020 if Trump is elected again? Will impeachment be something that Trump is worried about under those Congresses? Is he smart enough to care?
If real impeachment talks happen now or then, there would be an enormous media blitz against impeachment by conservative groups I believe. I think the Democratic party should anticipate this, if they truly want to pursue it. If a Democratic candidate were to not explicitly run on impeaching Trump, and then champions the cause after being elected could risk alienating moderate conversations / independents that elected them. The messaging will have to be simple, apolitical, resonate with voters, and consistent (something like, “preserving the integrity of the greatest democracy in the world”) in order overcome the optics of a vicious media campaign. The Democratic party will have to ask the question of whether that is good politics to explicitly run on impeaching Trump or if they should stick to campaigning on a check on his power. But to change their position on impeachment after being elected could hurt long term gains for the party.
If there is proof of treason/collusion with Russia (as opposed to preponderance), and it is perceived this way in the minds of the average voter, then impeachment is likely. But if it isn’t there broadly, I doubt impeachment is on the table. We’re only 100++ days in though. 1300 days to go!