> Whoever, being an officer of the Army or Navy, or other person in the civil, military, or naval service of the United States, orders, brings, keeps, or has under his authority or control any troops or armed men at any place where a general or special election is held, unless such force be necessary to repel armed enemies of the United States, shall be fined under this title or imprisoned not more than five years, or both; and be disqualified from holding any office of honor, profit, or trust under the United States.

* https://www.law.cornell.edu/uscode/text/18/592

Per DoJ guidance this also means that FBI agents cannot conduct investigations inside polling places on Election Day and deputy U.S. marshals cannot be stationed at open polls because they are armed while on duty:

* https://www.govinfo.gov/content/pkg/GOVPUB-J-PURL-gpo145659/...