SAN FRANCISCO — Senator Clinton is lending her voice, quite literally, to a campaign to defeat a California ballot initiative that would require teenage girls seeking abortions to notify a parent or get a waiver from a judge.

About 250,000 Californians received automated phone messages yesterday in which Mrs. Clinton warned about the dangers of the proposed parental notification law, according to opponents of the measure, which goes before voters next month as Proposition 85.

“We are opposed because 85 will put our most vulnerable teens at risk — teens who may already be endangered by negligent or even abusive homes,” Mrs. Clinton says in the audio recording. “We can do better. Let’s work together to protect all our children.”

Supporters of the measure contend that Mrs. Clinton’s active involvement in a campaign taking place thousands of miles from New York undercuts an effort she made early last year to articulate a more centrist stance on abortion in advance of a possible presidential bid in 2008.

“It looks like Hillary’s moving pretty far to the other side now,” a spokesman for the Yes on 85 effort, Albin Rhomberg, said. “It also shows that Hillary is quite fickle. She seems to want to please whatever the winds are or the tide at the time.”

A top aide to Mrs. Clinton, Ann Lewis, denied any recent adjustment to the senator’s stance on abortion or parental notification.

“Since 2000, when she ran for the Senate from New York, her position has been a consistent one,” Ms. Lewis said. “She feels strongly that young teens who would make such a decision should talk to their family and should have the benefit of parental counseling. In most cases, they do. We all know that in a few cases that is just not possible.”

In January 2005, Mrs. Clinton delivered a carefully calibrated speech to abortion rights advocates in Albany. In her remarks, she reaffirmed her staunch support for the right to terminate a pregnancy. However, she also expressed respect for those deeply opposed to abortion and referred to the procedure as “a sad, even tragic choice for many.” The senator also called for a renewed effort to reduce unintended pregnancies.

Mrs. Clinton’s aides insisted at the time that her stance was not new and was in keeping with President Clinton’s oft-stated goal of making abortion “safe, legal, and rare,” but the remarks led to a spate of stories about the senator’s effort to reformulate her views to make them more palatable to moderate voters.

While the speech led to some grumbling from abortion rights advocates, her remarks at a news conference the same day triggered a sharply negative reaction from the abortion rights lobby. While describing her history on the issue, she declared, “I supported parental notification with a judicial bypass,” according to a report in the New York Times. She was referring to a parental notification law she supported in Arkansas and endorsed as recently as 1992.

After abortion rights advocates expressed concern that Mrs. Clinton was wavering on parental notification, which some on both sides of the debate view as a step toward additional restrictions on abortion, her aides said she thinks the best approach is a New York law that requires a briefing on medical options but no notification to parents.

“The senator believes that New York is the model. In states where it is the only option, then yes, she supports parental notification with judicial bypass,” a spokeswoman for the senator, Jennifer Hanley, told the Times last year.

Asked yesterday how the California law Mrs. Clinton now opposes differs from the Arkansas statute she supported, Ms. Lewis said she was not familiar with the specific provisions. However, she said the option to avoid parental notification by seeking a judge’s permission does not resolve Mrs. Clinton’s concerns.

“You’re still saying again to exactly those teens who might be most vulnerable … they have to go to court,” Ms. Lewis said. “That’s a hard thing to do, especially when there are better ways.”

In one indication of the confusion over Mrs. Clinton’s position on the issue, backers of the California parental notification proposal list her on their Web site as favoring the concept.

“Parental notification has broad, bipartisan support — both Hillary Clinton and John Kerry have stated they support it,” the Yes on 85 site declares.

When told that the measure’s proponents suggest that they have Mrs. Clinton’s support, Ms. Lewis said, “I find that sort of breathtaking. That turns her position on its head. She supports parental involvement, families being involved with young people at such times. That is very different from that law.”

Just last fall, California voters defeated a similar parental notification measure, Proposition 73. About 47% of voters backed the measure, while 53% opposed it.

Ms. Lewis said Mrs. Clinton spoke out on the issue during a speech in Los Angeles last October.

A transcript of the senator’s remarks posted by the Women’s Foundation of California, which hosted the event, indicates that she spoke in general terms and never explicitly rejected the ballot measure.

“We obviously hope and expect that our children will come to us if they face difficult circumstances such as an unplanned pregnancy, but we also know that sometimes in the real world, families are in crisis, or there’s a history of violence, and young people simply cannot confide in their parents,” Mrs. Clinton said. “In situations like that, laws cannot mandate family communications, and there needs to be recognition and acceptance of that.”

In July, Mrs. Clinton voted against a federal bill that would make it a crime to take a minor across state lines to avoid a state parental notification requirement. The senator delivered an impassioned speech on the Senate floor recounting her involvement as a lawyer with teenagers from troubled homes in Arkansas, including some where pregnancy resulted from incest.

“I met the 12-year-old who was now pregnant with her father’s baby, and my heart just broke,” Mrs. Clinton said. “Now who was that child supposed to talk to? Where was that child supposed to go?”

A spokesman for the No on 85 campaign, Stephen Smith, said he expected Mrs. Clinton’s phone messages, known by political operatives as robocalls, to have an impact. “She has a name people recognize. People stand up and take notice,” he said.