A decision on whether a situation constitutes jury intimidation can be made in two different contexts, with different standards applied.
In the first case the question is whether the situation is such that one or more jurors should be dismissed from the jury, or perhaps the whole jury should be dismissed, and a mistrial declared. That will be a matter for the Judge to decide, and the question is whether jury members feel so threatened that it might or will affect their decision.
In the second case the question is whether some particular individual (or group) should be criminally charged. There the standard will be the normal "proof beyond a reasonable doubt" for any criminal case.
In Jury Reform in Wisconsin (May 2006) it is stated that
Two percent [of the Wisconsin judicial circuits] reported an incident of jury tampering or threatened jurors in the last five years.
I find Wisconsin laws against intimidation of witnesses and of crime victims, but neither includes jurors. Nor do I find the wsconson law against jury tampering, although the mention in the Jury Reform document makes it clear that there must be such a law.
I also found a news report of a case or perhaps just an allegation of jury tampering in Kenosha Wisconsin within the past few years. But the story is behind a paywall, and I could read only the lead paragraph, and that only for a few moments before the site transferred me to a subscription page.
The LII page on Jury Tampering reads:
Jury tampering refers to improper communications with a juror with the purpose of influencing the juror’s deliberative process via private communication or contact regarding matters directly related to the case being tried. Examples of jury tampering may include providing outside information to a juror and bribing, threatening or intimidating a juror to influence the verdict. Both lawyers and jurors themselves can be involved in jury tampering.
Jury tampering is not only an ethical infraction, but a criminal offense. Depending on states, jury tampering can be a felony offense or a misdemeanor. Penalties for jury tampering may also vary from states to states. ...
... The Supreme Court in Remmer v. United States held that jury tampering in criminal cases are presumptively prejudicial, meaning that the party faced with allegations of jury tampering bears the burden to prove that there is no reasonable possibility that the tampering affected the impartiality of the jury.
The US Legal page reads:
Jury tampering is the crime of attempting to influence a jury through other means than the evidence presented in court, such as conversations about the case outside the court, offering bribes, making threats or asking acquaintances to interfere with a juror.
A person commits the crime of jury tampering if, with intent to influence a juror's vote, opinion, decision or other action in the case, he attempts directly or indirectly to communicate with a juror other than as part of the proceedings in the trial of the case. Jury tampering may be committed by conducting conversations about the case outside the court, offering bribes, making threats or asking acquaintances to communicate with a juror.
One of these overviews stresses private communications, the other mentions one who "attempts directly or indirectly to communicate with a juror other than as part of the proceedings in the trial of the case."
It is not clear when or if protestors outside the courthouse would be guilty of jury tampering, since I cannot find the exact wording of the WI law on the subject. It would also be needed to establish an intent to influence jurors for criminal prosecutions. But if jurors felt threatened, the court could take action, which might involve dismissing a juror or requiring a retrial. It might also involve contempt of court citations, as ohwilleke points out in another answer.