The actual language is as follows:
Any citizen of the United States, wherever he may be, who, without authority of the United States, directly or indirectly commences or carries on any correspondence or intercourse with any foreign government or any officer or agent thereof, with intent to influence the measures or conduct of any foreign government or of any officer or agent thereof, in relation to any disputes or controversies with the United States, or to defeat the measures of the United States, shall be fined under this title or imprisoned not more than three years, or both.
This section shall not abridge the right of a citizen to apply, himself or his agent, to any foreign government or the agents thereof for redress of any injury which he may have sustained from such government or any of its agents or subjects.
It is legitimately thought that the wording is so broad as to encompass many expressions or communications protected by the First Amendment, such that the entire statute is unconstitutional or that it is unconstitutional as applied to many of the situations about which we fantasize prosecutions.
For example, the term "controversies" is used elsewhere in the Constitution, notably Article III, which extends the jurisdiction of US federal courts to "cases" and "controversies." Those have been interpreted to mean something actual, not theoretical, hypothetical, or "made up." US involvement in Ukraine probably constitutes a controversy. Just about any matter concerning US-Russia relations, about which people disagree, probably constitutes a controversy.