shows the 'Back to the Future' actor shaking and rocking uncontrollably, the visible signs of Parkinson's Disease, and speaking directly to camera as he makes the case for stem cell research by backing a Democratic candidate in the race for the Senate.
In the spot, Fox tells Missouri voters: "What you do in Missouri matters to millions of Americans. Americans like me."
The stem cell issue is an emotive one in American politics and is opposed by George W Bush. However, polls show that many favour some level of stem cell research and the issue could prove a winning one for Democrats in the mid-term elections.
President Bush vetoed a controversial bill that 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.
Fox was diagnosed with Parkinson's Disease, a chronic disorder of the central nervous system that leaves people unable to control their movements, in 1991, but did not reveal his condition publicly until 1998. Since then he has raised millions of dollars for research and has only acted sporadically since 2000 when he left the cast of comedy 'Spin City'.
The ad has been attacked by Republican supporters, and conservative radio commentator Rush Limbaugh has claimed Fox was "either off his medication or acting" during the spot.
Others have voiced concern that the ad could backfire as happened when Christopher Reeve campaigned for stem cell research before his death two years ago.
Claire McCaskill, a Democrat who is challenging Missouri's Republican Senator Jim Talent, who opposes research, for his seat is one of the candidates endorsed by Fox.
McCaskill said: "We all know someone who suffers from a debilitating disease or injury who may benefit from through stem cell research. I believe supporting stem cell research is supporting life."
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