SAN FRANCISCO – Another of Governor Arnold Schwarzenegger’s Hollywood friends took to the political stage yesterday, as actor and director Rob Reiner publicly denounced a ballot measure the governor is backing that would reduce the influence of public-sector unions in California.

“This is so divisive and so bad,” Mr. Reiner said of the so-called paycheck protection initiative, which would require unions to get written consent each year before using a member’s dues for political purposes. “It’s bad for our individual parochial interests, and it’s ultimately bad for democracy as a whole.”

Mr. Reiner spoke as he moderated a discussion with several dozen union members and Democratic activists assembled by a labor-funded group opposing the ballot measure. The acclaimed director, who has been mentioned as a possible challenger to Mr. Schwarzenegger in 2006, seemed intent on keeping the forum focused on the referendum, but one audience member took the floor to prod Mr. Reiner to jump in the race.

“I really wish you or Warren Beatty would run for governor, because people want movie stars or television stars,” a former city official, Doris Ward, said. “The way to beat Arnold is with Rob Reiner,” she said, before the actor interrupted.

“Is with a Meathead?” Mr. Reiner joked, referring to the character he played on the long-running television series “All in the Family.”

Ms. Ward made clear she was not kidding. “We need to get the governor out. I’m very serious about that,” she said.

Mr. Reiner continued the discussion without addressing his political ambitions.

After the forum concluded, reporters pressed the actor for a definitive answer about a possible challenge to Mr. Schwarzenegger.

“We’re not going to talk about that right now,” Mr. Reiner said, before making a swift exit.

Ms. Ward, a former member of the San Francisco Board of Supervisors, said in an interview that two Democrats already running for governor, state controller Steve Westly and state treasurer Phil Angelides, are “definitely more qualified” than Mr. Reiner. “But the public’s not looking at that. It’s sad, sad, sad,” she said. “I don’t think Steve or Phil or anyone outside of the profession of celebrity can beat Arnold.”

Mr. Reiner said earlier this month that he has not ruled out seeking public office, but is focused now on gathering signatures for a future ballot initiative to fund public preschools by raising taxes on wealthy Californians. In 1998, Mr. Reiner led a successful drive for a similar proposition that raised cigarette taxes for early childhood programs.