Julian P. Heicklen, a 79-year-old retired chemistry professor, has often stood on a plaza outside the United States Courthouse in Manhattan, holding a “Jury Info” sign and handing out brochures that advocate jury nullification, the controversial view that if jurors disagree with a law, they may ignore their oaths to follow it and may acquit a defendant who violated it.

Then, last year, federal prosecutors had Mr. Heicklen indicted, charging that his activity violated the law against jury tampering. Lawyers assisting him have sought dismissal of the case on First Amendment grounds.

But now prosecutors are offering their first detailed explanation for why they charged Mr. Heicklen, arguing in a brief that his “advocacy of jury nullification, directed as it is to jurors, would be both criminal and without Constitutional protections no matter where it occurred.”

“His speech is not protected by the First Amendment,” prosecutors wrote.

“No legal system could long survive,” they added, “if it gave every individual the option of disregarding with impunity any law which by his personal standard was judged morally untenable.”