On what grounds could threats of civil unrest overturn the results of a trial?

Question

On Saturday, April 17, 2021, United States Congresswoman Maxine Waters called for protesters to “stay on the street” and “get more confrontational” if former Minneapolis police officer Derek Chauvin is found guilty of only manslaughter in the ongoing trial regarding the death of George Floyd. This is in the wake of country-wide civil unrest that resulted in Floyd's death in May of 2020.

Waters' comments later prompted Chauvin's defense attorney Eric Nelson to urge Judge Peter Cahill to declare a mistrial, arguing that “an elected official, US Congressperson” made statements that “I think are reasonably interpreted to be threats against the sanctity of the jury process” and had the effect of “threatening and intimidating the jury.” While Cahill denied the request, he went on to say to Nelson “I’ll give you that Congresswoman Waters may have given you something on appeal that may result in this whole trial being overturned.”

On what grounds could the defense try and overturn a verdict under circumstances as described, and further, has anything like this happened in the past?

Answer 1

When the accused has not received a fair trial or due process

The right to a fair trial is guaranteed by the sixth amendment. The right to due process is guaranteed by the fourteenth. Justice Clark said in Sheppard v Maxwell, 384 U.S. 333 (1966):

This federal habeas corpus application involves the question whether Sheppard was deprived of a fair trial in his state conviction for the second-degree murder of his wife because of the trial judge's failure to protect Sheppard sufficiently from the massive, pervasive and prejudicial publicity that attended his prosecution

The appellant would need to demonstrate that Waters' comments (among other things) were such that they led the jury to consider matters other than the evidence in the trial. For what it's worth, I agree with the trial judge.

Answer 2

In the case of Powell v. Alabama, 287 U.S. 45 (1932) the hostility of the populace to the defendants was mentioned as increasing the case for overturning the verdict, but the main reason was the failure to supply counsel for the defendants, and the resulting lack of a fair trial.

The LII page on "Impartial Jury" says:

It is undeniably a violation of due process to subject a defendant to trial in an atmosphere of mob or threatened mob domination Frank v. Mangum, 237 U.S. 309 (1915); Irvin v. Dowd, 366 U.S. 717 (1961); Sheppard v. Maxwell, 384 U.S. 333 (1966).

In the opinion in the case of Frank v. Mangum, the Court wrote:

A request was made to poll the jury, and, just after the polling had begun, loud cheering from the crowd in the streets adjacent to the courthouse was heard. This cheering continued during the polling of the jury. The plaintiff in error insists that the cheering on the outside of the courtroom, which was loud, and which was heard by the jury, could not have been interpreted otherwise than as expressive of gratification at the verdict which had been rendered, and of which the crowd on the outside had in some way been informed, and was so coercive in character as to affect the fairness of the poll of the jury which was taken. . . . [P. 282.] In order that the occurrence complained of shall have the effect of absolutely nullifying the poll of the jury, taken before they dispersed, it must appear that its operation upon the minds of the jury, or some of them, was of such a controlling character that they were prevented, or likely to have been prevented, from giving a truthful answer to the questions of the court.

A defense motion in the case read, in part:

"The defendant was not in the courtroom when the verdict was rendered, his presence having been waived by his counsel. This waiver was accepted and acquiesced in by the court because of the fear of violence that might be done the defendant were he in court when the verdict was rendered."

The court further wrote:

Thus, the petition contains a narrative of disorder, hostile manifestations, and uproar which, if it stood alone and were to be taken as true, may be conceded to show an environment inconsistent with a fair trial and an impartial verdict.

...

We are very far from intimating that manifestations of public sentiment, or any other form of disorder, calculated to influence court or jury, are matters to be lightly treated.

...

The Georgia courts, in the present case, proceeded upon the theory that Frank would have been entitled to this relief [a new trial] had his charges been true, and they refused a new trial only because they found his charges untrue save in a few minor particulars not amounting to more than irregularities, and not prejudicial to the accused. There was here no denial of due process of law.

So a threat of a hostile populace, if it did or might have influenced the jury, or the judge, may constitute a denial of due process, and be grounds for overturning a conviction. But only where evidence of actual or probable interference with the judge or jury is found credible by the reviewing courts, as it was not in Frank.