...it's a pretty long leap to treason from where we currently stand.
"Whoever, owing allegiance to the United States, levies war against them or adheres to their enemies, giving them aid and comfort within the United States or elsewhere is guilty of treason and shall suffer death, or shall be imprisoned not less than 5 years and fined under this title but not less than $10,000.00; and shall be incapable of holding any office under the United States" - 18 US Code 2381
I guess it comes down to how you view things.
- Is Russia an enemy?
- Did Flynn's off the record conversation with the Russian ambassador where he said that he would help end/relax Obama's sanctions against them constitute aid and comfort?
If the answer is yes to both of the above, then it seems to be pretty clear cut.