Canadian-born actor Michael J. Fox has put himself in front of the cameras for the U.S midterm elections to help the election of Democrats who support embryonic stem cell research.

"What you do in Missouri matters to millions of Americans. Americans like me," Fox, who is now an American citizen, said in an ad for Claire McCaskill. She is running for the U.S. Senate in Missouri against an incumbent Republican.

The video, available on the online service YouTube, has been viewed more than one million times.

While actors routinely endorse various causes, Fox differs because he actually has Parkinson's disease.

The condition is a chronic, progressive nervous system disorder. Over time, the patient becomes increasingly unable to control their movements.

Doctors diagnosed Fox with Parkinson's in 1991 and he made his condition public in 1998. Two years after that, he quit full-time acting and created the Michael J. Fox Foundation for Parkinson's Research.

That organization has raised more than US$80 million to help Parkinson's sufferers.

Being a pitch man in the heat of a U.S. midterm election, however, means making one's self a target.

Conservative talk show host Rush Limbaugh opined that Fox was "either off his medication or acting" during the ad.

"In this commercial he is exaggerating the effects of the disease. He's moving around and shaking. It's purely an act," said Limbaugh.

Dr. John Boockvar, a New York professor and neurosurgeon, described Limbaugh's acting claim as "ludicrous." He noted that Parkinson's patients have on and off spells with respects to their symptoms.

After getting blasted for those remarks, Limbaugh adjusted his stance, saying there was "nothing admirable" about what he saw as Fox's exploitation of his condition.

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

Stem cells are unique because they can divide themselves for long periods, are unspecialized and can develop into specialized types.

Researchers think stem cells can replace the damaged nerve cells in the brain that lead to Parkinson's.

However, religious conservatives in the United States oppose embryonic stem cell research because the cells are taken from embryos that could develop into human beings.

"This bill would support the taking of innocent human life in the hope of finding medical benefits for others," U.S. President George Bush said in July when vetoing the Stem Cell Research Enhancement Act. "It crosses a moral boundary that our decent society needs to respect."

In Missouri, the political aspect of the stem cell controversy is heightened by a ballot issue -- Amendment 2: Missouri Stem Cell Research and Cures Initiative. Advocates of the initiative say it would protect the right of Missouri patients to have their injuries or illnesses treated by any stem cell therapy approved by federal law.

With the referendum and a tight race in Missouri between McCaskill and Talent, one political observer said Fox's ads could have an impact.

"If a tiny ad can change votes, this one ought to,'' said Larry Sabato of the University of Virginia's Center for Politics. "This is real. He's not playing a guy with Parkinson's -- he is a guy with Parkinson's.''

With files from The Associated Press