On Tuesday while in suburban Chicago campaigning for Democratic candidate for Congress Tammy Duckworth, Fox shot back.
"I'm kind of lucky right now. It's ironic, given some of the things that have been said in the last couple days, that my pills are working really well," Fox said.
A victim of Parkinson's disease, Fox speaks out in the ad for Missouri Democratic Senate candidate Claire McCaskill, who supports embryonic stem cell research.
"I think this is exploitative in a way that's unbecoming of either Claire McCaskill or Michael J. Fox," Limbaugh said on his syndicated show.
Duckworth added of Limbaugh's remarks: "That comment is trying to take away from the real issue at hand."
On his Web site Tuesday, Limbaugh appeared to back away from his accusation.
"All I'm saying is I've never seen him the way he appears in this commercial for Claire McCaskill," says Limbaugh. "So I will bigly, hugely admit that I was wrong, and I will apologize to Michael J. Fox, if I am wrong in characterizing his behavior on this commercial as an act, especially since people are telling me they have seen him this way on other interviews and in other television appearances."
"As you might know I care deeply about stem cell research," says Fox, who has struggled with Parkinson's disease for more than a decade. "In Missouri you can elect Claire McCaskill, who shares my hope for cures."
Katie Couric interviews Michael J. Fox about the stem cell debate on Thursday's CBS Evening News.
Watch Claire McCaskill's ad featuring Michael J. Fox here. Fox Responds To Limbaugh Claim.
McCaskill has made support for the research a key part of her campaign to unseat Sen. Jim Talent. The Republican incumbent opposes the research as unethical, saying it destroys human embryos.
The new ad debuted prominently Saturday night during Game 1 of the World Series between the St. Louis Cardinals and the Detroit Tigers and will continue airing statewide this week, a campaign spokeswoman said.
Debate over stem cell research looms large in Missouri, where voters are considering a ballot measure that would amend the state constitution to protect all federally allowed forms of the research, including embryonic stem cell research. The scientific study holds promise in the search to cure diseases like Parkinson's and Alzheimer's.
"Unfortunately, Senator Jim Talent opposes expanding stem cell research," Fox says in the 30-second spot. "Senator Talent even wanted to criminalize the science that gives us a chance for hope."
Talent's campaign called the ad a false attack.
"Senator Talent supports medical research including stem cell research that doesn't involve cloning or destroying a human embryo," said Talent spokesman Rich Chrismer.
Earlier this year, Talent withdrew his support for a Senate bill that would ban all embryonic stem cell research and impose a million-dollar fine and jail sentence on violators. But he opposes the Missouri ballot initiative, claiming it would "make cloning human life at the earliest stage a constitutional right."
Supporters of the state referendum deny that assertion, noting the language of the proposed constitutional amendment explicitly bans human cloning.
McCaskill, running even with Talent in the latest polls, hopes to get a boost from aligning herself with support for the ballot measure, which recent polls show has the support of 58 percent of likely voters. Backers of the measure have raised nearly $29 million toward its passage, the costliest political campaign in Missouri history.
Fox's ad urges voters that "what you do in Missouri matters to millions of Americans. Americans like me." The ad ends with a picture of McCaskill and her voice approving the message.
A native of Canada, Fox became an American citizen in 2000 and has been politically active for Democratic causes. He campaigned for John Kerry in the 2004 presidential race and lobbied Congress to lift President Bush's restrictions on funding for stem cell research.