Emphasis mine, except where noted.
The President referred to Adam Schiff, House Intelligence Committee chairman and prominent figure in the House's impeachment inquiry, saying:
He should resign from office in disgrace. And frankly, they should look at him for treason because he is making up the words of the president of the United States - not only words, but the meaning. And it's a disgrace. It should not be allowed to happen.
The President's remarks are dangerous, and may themselves approach the act of treason, inasmuch as they imply that the mere act of interpreting the speech of the President is a crime against the nation. In fact, in writing down these thoughts on the President's insipid drivel, I expose myself to the implicit threat.
Theodore Roosevelt said in a rebuttal to criticism of his remarks about then-President Woodrow Wilson:
To announce that there must be no criticism of the President, or that we are to stand by the President, right or wrong, is not only unpatriotic and servile, but is morally treasonable to the American public.
Writing in The Federalist, Alexander Hamilton drew a clear delineation between the actions of the King of Great Britain and the public servant of the President of the United States:
The President of the United States would be an officer elected by the people for four years; the king of Great Britain is a perpetual and hereditary prince. The one would be amenable to personal punishment and disgrace; the person of the other is sacred and inviolable.
In saying "It should not be allowed to happen," the President of the United States is threatening a Constitutionally enumerated power of the Legislative branch, as well as the protected right of free speech, the cornerstone of America's civil identity. Consider this and reflect on the definition of the crime of Treason, given by 18 U.S. Code § 2381 as:
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 five years and fined under this title but not less than $10,000; and shall be incapable of holding any office under the United States.