When Kat Sullivan rented a billboard last year in upstate New York to call for stro nge r protections against child sex abusers, she believed she was engaging in the democratic process, using her own time and money to make her voice as an abuse survivor heard.

So she was shocked when state regulators afterward sent her a letter ordering her to register as a lobbyist.

New York State defines a lobbyist as, in part, someone who spends money to influence lawmakers. But Ms. Sullivan, a registered nurse, has argued that she was exercising her rights as a citizen.

She is now locked in a battle with the state’s ethics commission, which has warned that she could be guilty of a misdemeanor and fined more than $40,000 if she continues to refuse to register .