"This kind of brazen and self-serving political stunt doesn't even deserve a response," he said in a written statement.
Newsom acknowledged the affair in a February forum moderated by Politico reporter Carla Marinucci at which he applauded the #MeToo movement for prompting thousands of women to share their stories of sexual harassment and assault.
“I acknowledged it. I apologized for it. I learned an enormous amount from it," Newsom said last month. "And I am, every day, trying to be a champion and a model," for girls and boys.
The woman at the center of his affair, Ruby Rippey Gibney, spoke out shortly after the issue arose in February, posting on her Facebook page that she doesn't blame anyone else for what she called an "ugly episode," and that while she fully supports the #MeToo movement, she has "doubts" it applies to her situation.
"Yes, I was a subordinate, but I was also a free-thinking, 33-yr old adult married woman & mother. (I also happened to have an unfortunate inclination towards drinking-to-excess & self-destruction.)," she wrote.
Renteria, who stepped down as chief of operations for the California attorney general, pushed back on Twitter when some people suggested she was conflating consensual sex with harassment.
"Workplace impacts of this type of behavior has real implications on the lives of everyone in the office. The more power you have, the more responsibility you have to ensure a positive culture where everyone is empowered," she wrote.
Newsom isn't the only gubernatorial candidate who has been plagued with questions about his past.
Former Los Angeles Mayor Antonio Villaraigosa had an affair with a television reporter who covered his administration while he held the post. Villaraigosa was married at the time. And Republican candidate Travis Allen, a state assemblyman, was accused of inappropriately touching a women in 2013. A legislative committee found the accusation substantiated.
Several prominent Democratic women immediately jumped to Newsom's defense, including consultant and former party staffer Shawnda Westly.