In my Googling, I learned that (at least in most of the U.S.) a defendant cannot be an accessory to a misdemeanor. What about someone just nagging and insisting on it?
This is called solicitation.
A person is guilty of solicitation to commit a crime if with the purpose of promoting or facilitating its commission he commands, encourages or requests another person to engage in specific conduct which would constitute such crime.
Previously, the offence was inciting the commission of another offence contrary to Common Law which was abolished by s.59 of the 2007 Act.
Just encouraging someone to commit a crime can make one an accomplice. See, e.g., People v. Prettyman
an aider and abettor must act with knowledge of the criminal purpose of the perpetrator and with an intent or purpose either of committing, or of encouraging or facilitating commission of, the offense.
You didn't tag your question with some country.
So here is my answer for germany:
In Germany, §26 StGB states:
Als Anstifter wird gleich einem Täter bestraft, wer vorsätzlich einen anderen zu dessen vorsätzlich begangener rechtswidriger Tat bestimmt hat.
The word "Anstiftung" translates to "incitement" or "instigation".
Freely translated, the law means:
A person who deliberately instigates another person to deliberately perform an illegal act will be punished the same way as the person who performed the illegal act.