The answer to this question can be found in Article III, Section 3 of the Constitution of the United States.
First, a person has to commit an act that fits the definition of treason that is given in the Constitution. Such acts are limited to:
- Making war against the United States
- “Adhering” to its enemies.
- Giving “aid and comfort” to its enemies.
We should note that this is a very vague set of criteria for what could be defined as treason. It does not define what making war is, what an enemy is, or what it means to adhere to or give aid and comfort to an enemy.
Second, the person cannot be convicted unless they make a confession in open court or if there are two witnesses who can both testify to the same overt act.
The Constitution meant to make it difficult to convict anyone of treason so it would not be used as a charge to be made against political enemies.