Lol, they're called high crimes and misdemeanors. And they are whatever Congress decides they are. Anything that would otherwise be a criminal charge is an obvious case. Impeachable.

A number of his inner circle are already in it up to their eyeballs. Lying on the US government security clearance (SF-86) about contacts with foreign officials is a federal offense. They cut Flynn away right off the bat but it looks like the entire lot only did a slightly better job of covering it up.

It only remains to be seen if it amounts to election fraud and influence peddling. If there was a promised exchange for Russian information or activity during the election that's straight up jail time. Like acting in the interests of a foreign government, treason jail time.