The plea is as disturbing — and arresting — as a hostage video from Iraq. In a navy blazer and preppy Oxford shirt, the actor Michael J. Fox calmly asks viewers to support stem cell research by voting for several Democratic candidates in Maryland, Missouri and Wisconsin, while his body sways back and forth uncontrollably like a sailor being tossed around in a full-force gale.

In short, Mr. Fox’s display of the toll Parkinson’s disease has taken on him turned into one of the most powerful and talked about political advertisements in years.

Republican strategists who saw how quickly the commercial was downloaded, e-mailed and reshown on news broadcasts certainly thought so. Rush Limbaugh rushed in to discredit Mr. Fox, though he mostly hurt himself. Mr. Limbaugh, the conservative radio talk show host, told his listeners that the actor either “didn’t take his medication or was acting.” Mr. Limbaugh later apologized for accusing Mr. Fox of exaggerating his symptoms, but said that “Michael J. Fox is allowing his illness to be exploited and in the process is shilling for a Democrat politician.”

Republicans cobbled together a response ad that did not mention Mr. Fox but attacked the ethics of embryonic stem cell research. It included testimonials by the actress Patricia Heaton (“Everybody Loves Raymond”) and James Caviezel, who played Jesus in Mel Gibson’s “Passion of the Christ.” At least in the advance version shown on YouTube last night, Mr. Caviezel’s introduction seemed either garbled or to be in Aramaic.