It's an impeachable offense, although whether or not it'd qualify as treason would be interesting law debate.Treason is not even up for debate.
Treason is SPECIFICALLY defined in the Constitution requiring the nation to be at war with the 'enemy' who is given aid.
Unless we are at war with Russia -- and last time I checked we were not -- anything involving Russia cannot be treason.
Even though there is no formal declaration of war, it is possible that a short list of countries could be considered at war due to ongoing Congress-authorized military actions: Pakistan, Somalia, Yemen, Afghanistan, Syria, Iraq and Libya. If he were caught giving aid to any of these countries then there is at least the possibility of treason.
Since we're not currently at war with Russia, any aid or comfort given to the country or Russian citizens can't be treason under the Constitution.
This is the interesting part. Because we're not at war with China, but just recently someone was found passing information along to them. Then there was the scientist before that (Los Alamos, IIRC). Most civilians would call that treason. But if passing gov't info to a foreign or hostile power isn't treason, then what is it?
Would you have Trump impeached? The Constitution only allows for three reasons of impeachment; Treason (defined as giving material aid and comfort to enemies we are at war with), Bribery, and Maladministration (called "High Crimes and Misdemeanors" in that era, but understood to be the term of the era for maladministration). Since he wasn't in office at the time, neither Bribery nor Maladministration could apply.
Ok, technically his collusions prior to getting the High Office isn't impeachable. But lying about it once he's in office is. That would fall under High Crimes and Misdemeanors, I would think.