Actor Michael J Fox, who has Parkinson's Disease, is lending his support to US Congressional candidates who advocate stem cell research.
Fox, 45, has appeared at a rally and filmed campaign adverts for Democrats who support research on human embryos.
The adverts show the Back To The Future star rocking uncontrollably, one of the effects of his condition.
Scientists hope embryonic stem cell research could one day yield cures for illnesses like Parkinson's or diabetes.
Opponents, led by the Catholic Church and anti-abortionists, say it is immoral and could lead to human cloning.
President George W Bush is also against the research, and vetoed a controversial bill which would have lifted a ban on federal funding for embryonic stem cell research in July.
"It crosses a moral boundary that our decent society needs to respect, so I vetoed it," he said.
However, polls suggest the majority of Americans back the research, and it has become a key issue in the mid-term elections, which decide the make-up of Congress, the two houses of the US legislature.
In Missouri, voters will be asked to decide on "Amendment Two" - changing the state constitution to permit federally-approved research on cells derived from human embryos.
Claire McCaskill, a Democrat who is challenging Republican Senator Jim Talent for his seat in the state, is one of the candidates endorsed by Fox.
"What you do in Missouri matters to millions of Americans. Americans like me," the actor says in a campaign advert.
"Senator Jim Talent opposes extending stem cell research. Senator Talent even wanted to criminalise the science which gives us a chance for hope."
Scientists say stem cell research could combat Alzheimer's and Parkinson's
Fox's adverts have attracted criticism, with conservative radio commentator Rush Limbaugh claiming he was either "off his medication or acting" in the 30-second clip.
The Missouri Senate race is one of the most closely-watched contests in an effort by US Democrats to capture control of Congress in the mid-term elections, which take place on 7 November.
Larry Sabato, director of the Centre for Politics at the University of Virginia, said Fox's intervention could help decide the race.
"If a tiny ad can change votes, this one ought to," he said.
"This is real. He's not playing a guy with Parkinson's - he is a guy with Parkinson's."
Parkinson's is a degenerative disease of the nervous system that generally affects both men and women who are more than 40 years old.
Fox is best remembered for the Back To The Future trilogy
The disease develops slowly over time and is associated with trembling of the arms and legs, stiffness and rigidity of the muscles and slowness of movement.
A third of people with Parkinson's also develop senile dementia.
People with the disease eventually die from secondary complications such as pneumonia, urinary tract infection, pressure sores, septicaemia and stroke.
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, which has raised millions of dollars for research into a cure.