New York ? The symptoms of Parkinson’s disease that all but ended Michael J. Fox’s acting career are making him a powerfully vulnerable campaign pitchman for five Democrats who support stem cell research.

In 30-second TV ads for Rep. Benjamin L. Cardin, who is running for the Senate in Maryland, Senate candidate Claire McCaskill in Missouri and Wisconsin Gov. Jim Doyle, Fox shakes and rocks as he directly addresses the camera, the effects of his disease clearly apparent.

In the McCaskill ad, which has been viewed by more than 1 million people on YouTube.com, Fox tells voters, “What you do in Missouri matters to millions of Americans. Americans like me.”

Fox was considering appearing with several political candidates who support stem cell research around the country.

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 embryonic stem cell research, which some scientists believe could aid in discovering treatments or cures to Parkinson’s and other diseases.

“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.”‘

Actor Michael J. Fox listens to Tammy Duckworth, the Democratic candidate for the U.S. House in Illinois' 6th District, during a rally for stem cell research in Wheaton, Ill. Fox is campaigning for several other candidates, including Claire McCaskill, of Missouri, in a race for a U.S. Senate seat.

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

Fox, 45, was diagnosed with Parkinson’s in 1991 and revealed his condition publicly in 1998. In 2000, the “Spin City” and “Back to the Future” star 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.”

Some have criticized Fox’s ads as exploitive. Conservative radio commentator Rush Limbaugh claimed Fox was “either off his medication or acting” during the ad.

Others defend Fox’s aggressive campaigning for a Parkinson’s cure.

Dr. John Boockvar, a neurosurgeon and assistant professor at Weill Cornell Medical Center at New York’s Presbyterian Hospital, called Limbaugh’s claim that Fox was acting “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.”