A group of people tried to overthrow the government and interrupt the peaceful transfer of power in order to install their leader as president. Those people could be accurately described as enemies of the nation. Senator Vance here is saying that, if he had had the opportunity, he would have seen that those people were successful. Vance has sworn an oath to support and defend the Constitution of the United States. Isn't this giving aid and comfort to the nation's enemies? That pretty much meets the definition of treason if so.