Video of the incident was captured by onlookers and the teenage girl can be seen punching the man before the attack.
Thousands had gathered outside a hotel with pro-Trump supporters on one side and anti-Trump demonstrators on the other.
The protest was planned before Trump's rally took place in Janesville. Police say it's "currently looking for two suspects, one for the sexual assault and one for the pepper spray."
Suggested For You
SUBSCRIBE AND FOLLOW
Get top stories and blog posts emailed to me each day. Newsletters may offer personalized content or advertisements.Learn more