2016 Postmortem

There is nothing that would prevent Democratic members of the House and Senate from introducing a resolution formally declaring war on Russia.

For Trump to fit the Constitutional definition of a traitor, he must either have been levying actual war (i.e., taking up arms) against the United States. Or he must have been giving aid and comfort to an enemy. While enemy is not defined in the Constitution, it is defined elsewhere in the United States Code: the term "enemy" means any country, government, group, or person that has been engaged in hostilities, whether or not lawfully authorized, with the United States." The term "hostilities" is defined as "any conflict subject to the laws of war."

So if you want to go after Trump for "treason" the first step is for the US to make Russia an enemy for purpose of the treason clause by engaging in a conflict subject to the laws of war. And the clearest way to do that is for Congress to declare that a state of war exists with Russia.

I should add that a question that has been debated without clear resolution is whether a president could veto a declaration of war...