“[A]ll civil Officers of the United States, shall be removed from Office on Impeachment for, and Conviction of, Treason, Bribery, or other high Crimes and Misdemeanors. US Const, Art. II, Sec. 4.”
I think it would be a stretch for a candidate not holding government position at the time to be defined as a civil officer. Clearly the incumbent would be but an opponent completely divorced from government would not seem to be.I say that because the thrust of the bribery provisions is to discourage civil officers from trading on their positions in government and the contacts they make during their service to advance their interest. Obama would not be directly permitted to pay for info from Putin but Trump because he was not in government and thus he would not have made contact with Russia as a result of discharge of government duties. Having said that, the reality is they all pay for negative info even info coming from foreign sources through layers of campaign staff on one side and layers of foreign government agents on the other and keep the candidate insulated from the activity. Lastly how do we define an enemy? The political reality is right now Russia is called as an enemy by many. As a matter of diplomacy State does not call them an enemy. Others see them more as a irritating pain in the butt of no real threat of directly attacking the US.Is an enemy a country we do not like because their are many of those on the globe. Personally I think there currently are truly only 3 enemies of the US right now that can lead to direct confrontation NK, Iran and lesser extent China. All others range from friends like Israel to irritating pains in the ass like Russia.
He's not yet a civil officer, but I think original intent could construe that once he became on officer, the bribery by him as a candidate would be grounds for impeachment. Of course it wouldn't matter his status if he committed treason. The Constitution calls treason a crime regardless of his status.