NEW YORK -- The campaign ad opens with a familiar boyish face, now atop a body that sways uncontrollably. Michael J. Fox, wearing a shirt and suit jacket, talks directly to the camera.

• Video: Watch Fox Ad

"They say all politics is local, but it's not always the case," Fox says in the 30-second commercial backing Senate candidate Claire McCaskill in Missouri, a Democrat. "What you do in Missouri matters to millions of Americans -- Americans like me."

Fox, who suffers from Parkinson's disease and supports research on embryonic stem cell for a potential cure, also has lent his celebrity to Democrats Rep. Benjamin L. Cardin, running for the Senate in Maryland, and Wisconsin Gov. Jim Doyle, who is seeking re-election. Both politicians also back stem cell research.

The ads have triggered a backlash, with some criticizing them as exploitive. Conservative radio commentator Rush Limbaugh has claimed Fox was acting.

"This is really shameless, folks. This is really shameless of Michael J. Fox. Either he didn't take his medication or he's acting, one of the two," Limbaugh said.

"He is an actor, after all," Rush said later.

"He is exaggerating the effects of the disease," Limbaugh told listeners. "He's moving all around and shaking and it's purely an act."

"This is the only time I've ever seen Michael J. Fox portray any of the symptoms of the disease he has," Limbaugh said. "He can barely control himself."

Later he apologized on air.

"Now people are telling me they have seen Michael J. Fox in interviews and he does appear the same way in the interviews as he does in this commercial," Limbaugh said on his Web site. "All right then, I stand corrected. . . . So I will bigly, hugely admit that I was wrong, and I will apologize to Michael J. Fox, if I am wrong in characterizing his behavior on this commercial as an act."

Dr. John Boockvar, a neurosurgeon and assistant professor at Weill Cornell Medical Center at New York's Presbyterian Hospital, called Limbaugh's claim "ludicrous." Boockvar said those with Parkinson's have "on" and "off" spells.

"If there is one single disease that has the highest potential for benefit from stem cell research," Boockvar said Tuesday, "it's Parkinson's."

Celebrities have a long history of supporting political candidates. But there's no question that Fox, who campaigned for John Kerry in the 2004 presidential race, is uniquely suited as a spokesman for stem cell research.

Fox, 45, who starred on TV's "Family Ties" and "Spin City" plus the "Back to the Future" films, shakes and rocks as he directly addresses the camera, the effects of his disease clearly apparent.

"The reason that he's powerful is that he's comparatively young," says Kathleen Hall Jamieson, director for the University of Pennsylvania's Annenberg Public Policy Center. "As a result, a lot of people in that age range can look at him and say, `If that can happen to him, it can happen to me."'

Jamieson notes that the stem cell issue has the potential to be an advantage to Democrats in the upcoming midterm elections since polls have shown the majority of Americans favor some form of stem cell research. Critics say it requires the destruction of a human embryo.

The risk, Jamieson adds, is that the ads could appear as using Fox's hopes for a cure for political gain, as some claimed was the case when the paralyzed actor Christopher Reeve lobbied for stem cell research before his death in 2004.

Parkinson's disease is a chronic, progressive disorder of the central nervous system that leaves patients increasingly unable to control their movements.

Fox was diagnosed with Parkinson's in 1991 and revealed his condition publicly in 1998. In 2000, he quit full-time acting because of his symptoms and founded the Michael J. Fox Foundation for Parkinson's Research, which has raised millions of dollars.

He has since acted sporadically in smaller roles, such as in a several-episode guest appearance earlier this year on ABC's "Boston Legal," playing a business tycoon with cancer.

For that role and others, Fox generally has sought to control his movements, though his illness was evident. He told The Associated Press in January that one long scene was physically taxing and that because of Parkinson's disease, he "can't show up with a game plan."