That will be the line of Republicans under assault from what could become one of the most powerful political advertisements ever made. The new ad features an ailing Michael J. Fox talking about politicians who oppose embryonic stem-cell research. This is not just another celebrity ad, like those cut by the late Christopher Reeve. It’s a celebrity shot to the solar plexus of the GOP. Whatever happens in the campaign, the ad is already a classic and will be mentioned in the same breath as LBJ’s famous 1964 “Daisy” ad and other unforgettable political moments on television.

Rush Limbaugh helped cement the ad’s place in history with his astonishingly insensitive remark that Fox “was either off his medication or was acting.” Limbaugh quickly apologized but the damage to his own reputation was already done.

Fox, star of megahit TV shows and movies like “Family Ties” and “Back to the Future,” was for years one of the most popular actors in the United States. He still works, but is clearly debilitated by Parkinson’s. Throughout the ad, he sways back and forth, showing signs of advanced disease.

In the version cut for Democrat Claire McCaskill, who is running against Sen. Jim Talent in Missouri , the actor, wearing a blue blazer and open-collared shirt, says, “Senator Talent even wanted to criminalize the science that gives us a chance for hope.” This is in apparent reference to Talent’s early support for Sen. Sam Brownback’s view that embryonic stem-cell research should be illegal. Then comes the clincher: “They say all politics is local, but it’s not always the case. What you do in Missouri matters to millions of Americans—Americans like me.”

The problem with the “poor taste” defense is that for anyone who has suffered from diseases that could some day be cured by such research—the ones you might expect to be appalled—the ad is not in the slightest bit offensive. It’s dead-on. Literally. It’s the “pro-cure” spot those of us suffering from serious diseases have been waiting for—the one that might hasten the political change that could give us more life. (In 2004, I received an adult stem-cell transplant for lymphoma and achieved remission , but in the long term, adult stem-cell treatments are not generally considered curative).

The political punch of this commercial and others like it (full disclosure: my sister has been involved in other stem-cell ads) could prove decisive in close contests. After all, people suffering from diseases like Parkinson’s, cancer, diabetes, Alzheimer’s and spinal-cord injuries have lots of friends and relatives. Many are on the fence in the election—they’re Republicans or independents. This could well tip them into the Democratic column in those races where the Republican stands in opposition to medical advancement.

In Missouri, former Sen. John Danforth, a well-respected Republican and former minister, told the Chicago Tribune that he was not supporting either his fellow Republican Talent nor McCaskill. The reason was their difference on stem cells.

Stem-cell ads have been already used in Wisconsin for Gov. Jim Doyle and in a half dozen House races. In Dupage County, Ill., Tammy Duckworth, who lost both legs in Iraq, will be joined Wednesday by Fox on the trail. Duckworth is facing an anticure Republican, Peter Roskam. I interviewed Roskam recently and he gave me the same line on stem cells that I heard last week from Rep. Mike Ferguson, another opponent of the research. Try adult stem cells, they say, and leave embryonic stem-cell research to the private sector. Unfortunately, adult stem cells, however promising, do not offer the same potential for cures right now. And the private sector has never funded much basic research in this country. In other words, no federal support for embryonic stem-cell research means it will take longer to find cures.

Supporters need to realize that these cures are not just around the corner. This week, for instance, a new study showed that embryonic stem cells injected into the brains of mice ended the symptoms of Parkinson’s, but left tumors. Much more work is required. But it’s fair to judge politicians on a simple standard: do they want progress toward cures—or not?

That’s a fair shot, and one in very good taste.