Well this violates 18 U.S. Code § 115.

(B)threatens to assault, kidnap, or murder, a United States official, a United States judge, a Federal law enforcement officer, or an official whose killing would be a crime under such section,

with intent to impede, intimidate, or interfere with such official, judge, or law enforcement officer while engaged in the performance of official duties, or with intent to retaliate against such official, judge, or law enforcement officer on account of the performance of official duties, shall be punished as provided in subsection (b).

(4)A threat made in violation of this section shall be punished by a fine under this title or imprisonment for a term of not more than 10 years, or both, except that imprisonment for a threatened assault shall not exceed 6 years.

Like, that’s textbook. Also if this threat is taken seriously by other members in the trial, this could easily extend into witness tampering, jury intimidation.