Witness intimidation is prohibited by 18 U.S.C. 1512, which imposes penalties on anyone who:
(a)(2) uses physical force or the threat of physical force against any person, or attempts to do so, with intent to ... influence, delay, or prevent the testimony of any person in an official proceeding
or
(b) knowingly uses intimidation, threatens or corruptly persuades another person, or attempts to do so, or engages in misleading conduct toward another person, with intent to influence, delay or prevent the testimony of any person in an official proceeding
Courts explicitly disclaim the use of bright-line tests in for violations of Section 1512 in cases like this, which do not involve the actual use of physical force. Instead, the courts look to the context of the speech or conduct in question to determine whether the defendant used a "threat of physical force" or "corruptly persuaded" the witness not to testify.
The guidance that the courts give on those questions is fairly squishy.
Under subsection (a)(2), for instance, threats of physical force would have to be "true threats" to satisfy First Amendment analysis, meaning that they were “serious expressions conveying that a speaker means to commit an act of unlawful violence.” Counterman v. Colorado, No. 22-138, 9 (U.S. Jun. 27, 2023). And there is no bright-line test to determine whether a threat is such a serious expression of intent to commit unlawful violence; instead, courts must look at “all of the contextual factors” that could inform that answer. Virginia v. Black, 538 U.S. 343, 345 (2003).
Subsection (b) appears to require the same totality-of-the-circumstances analysis rather than using any bright-line test:
There is general agreement that it does not require "acts, threats, emotional appeals, or persistent pleading," but does require the Government to prove "a defendant's action was done voluntarily and intentionally to bring about false or misleading testimony with the hope or expectation of some benefit to the defendant." ... However, the persuasion "need not be explicit" to be corrupt. ... Indeed, corrupt persuasion includes situations where a defendant coaches or reminds witnesses by [**18] planting misleading facts.
United States v. Edlind, 887 F.3d 166, 173-174 (2018).
Therefore, the Fourth Circuit upheld the conviction in Edlind even where the defendant explicitly told the victim to tell the truth:
Edlind's statements can be seen as attempts to confuse Kwiatkowski as to what was real and what was Chujoy's odd sense of humor. By confusing Kwiatkowski, Edlind sought to undermine his ability to testify persuasively against Chujoy in the Inca's Secret case. Coupled with the circumstantial evidence implicating Edlind -- the timing of events following Chujoy's June 3 letter and Edlind's constant attempts at avoiding governmental surveillance -- there is sufficient evidence to support the conviction for witness tampering. The jury was free to reject Edlind's statement to "tell the truth" as a shallow attempt to immunize herself from prosecution.