Missouri became the national battleground Wednesday in the embryonic stem cell research debate, as celebrity opponents of a state amendment — including a World Series starting pitcher — said they would air a commercial during the game in response to an ad in which actor Michael J. Fox graphically displays the symptoms of his Parkinson's disease.

The minute-long ad opposing Amendment 2, a Missouri ballot initiative that would protect stem cell research, features St. Louis Cardinals pitcher Jeff Suppan, who is scheduled to start Wednesday night's Game 4 against the Detroit Tigers.

Arizona Cardinals quarterback Kurt Warner, Kansas City Royals player Mike Sweeney and actors Patricia Heaton of TV's "Everybody Loves Raymond," and Jim Caviezel, who portrayed Jesus in "The Passion of the Christ" also are featured in the ad.

"Amendment 2 claims it bans human cloning, but in the 2,000 words you don't read, it makes cloning a constitutional right," Suppan says in the commercial. "Don't be deceived."

The ad comes in response to a series of controversial 30-second Fox ads, in which he asks voters to support various Democratic candidates and legislation backing embryonic stem cell research.

Those ads ignited a firestorm Tuesday when conservative talk-show commentator Rush Limbaugh told his listeners that the TV and movie star was "either off his medication or acting."

In the ads, Fox, 45, who was diagnosed with Parkinson's in 1991 and went public with the disease in 1998, shakes and rocks uncontrollably as he directly addresses the camera, making no effort to hide the effects of his disease.

Click here to view Michael J. Fox ad supporting McCaskill and 'Amendment 2'

Click here to view celebrity ad opposing Missouri ballot initiative 'Amendment 2'

Fox's Missouri ad asks voters to support Democratic Senate hopeful Claire McCaskill, who is campaigning to unseat Republican incumbent Jim Talent.

McCaskill supports the Missouri embryonic stem cell research amendment, while Talent is opposed.

Limbaugh weighed in on the Fox ad during his broadcast Tuesday.

"He is exaggerating the effects of the disease," Limbaugh told his listeners. "He's moving all around and shaking and it's purely an act. ... This is really shameless of Michael J. Fox. Either he didn't take his medication or he's acting."

In addition to backing McCaskill, Fox's ads have been airing in support of Rep. Benjamin L. Cardin, who is running for the Senate in Maryland, and Wisconsin Gov. Jim Doyle.

Fox also was planning to appear at events for Sen. Robert Menendez of New Jersey and Tammy Duckworth, a candidate for Congress from Illinois.

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In the ad supporting McCaskill, which by Tuesday night had been viewed more than 1 million times on YouTube.com, Fox tells voters, "What you do in Missouri matters to millions of Americans. Americans like me."

"All stem cell research is legal today in Missouri," Limbaugh countered. "Jim Talent does not seek to criminalize it, as Michael J. Fox asserts in his television commercial. The truth is, Amendment 2 would put human cloning in the Missouri Constitution. Michael J. Fox is participating in this disinformation campaign."

In addition to providing state constitutional protections for embryonic stem cell research, Amendment 2 also would ban cloning and egg harvesting for stem cell research.

Limbaugh asserts, however, that language in the bill — a reference to somatic cell nuclear transfer — is just another term for cloning.

"The fine print creates a right to do somatic cell nuclear transfer which is the scientific term for cloning, the same method used to clone Dolly the sheep!" Limbaugh charged.

Talent, the Republican incumbent, opposes the amendment on the grounds that he equates embryonic stem cell research with human cloning.

Talent's spokesman, Rich Chrismer, told USA Today that Fox's statements were "false," and said Talent supports "stem cell research that doesn't involve cloning or destroying a human embryo."

"Democrats cannot look you in the eye and say, 'Here's what we're for' and convince you to vote for it. They have to trick you, and the Michael J. Fox commercial in Missouri and Maryland is just the latest incarnation," Limbaugh charged.

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"Ludicrous," is how John Boockvar, a neurosurgeon and assistant professor at Weill Cornell Medical Center at New York's Presbyterian Hospital, described Limbaugh's claim that Fox was acting.

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

Boockvar said those with Parkinson's have "on" and "off" spells.

There's no question that Fox, a popular TV and movie actor who also 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.

"I'm a one-issue guy. I'm about stem cells," Fox told the St. Louis Post-Dispatch earlier this month during a campaign appearance for McCaskill that raised almost $200,000 for the state auditor's senate bid.

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

"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.'"

The risk in airing the ads is that they could appear as using Fox's hopes for a cure for political gain, Jamieson said.

Limbaugh made a similar point on his Tuesday program.

"I don't care what anybody says; it is unseemly, it is exploitative, and it is downright mean to mislead people who suffer from horrible diseases that there is a cure around the corner — if only Republicans could be defeated," Limbaugh said.

Larry Sabato, director of the Centre for Politics at the University of Virginia, told the BBC that 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."

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 about $80 million for research and education.

Fox 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."

The Associated Press contributed to this report.

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