The petition drive was announced last month by a group of business leaders, patient advocates and researchers as a response to legislative efforts to ban a type of stem cell research known as therapeutic cloning.
Missouri's Catholic dioceses oppose it, and urged their priests statewide to begin a campaign Sunday aimed at keeping Catholics from signing the petition.
At St. Peter Catholic Church, across the street from the state Capitol, the Rev. James Smith quoted Nazi propaganda chief Joseph Goebbels and compared the cultivation of human embryos for research to the gruesome experiments performed on concentration camp occupants during World War II.
"The similarities of the arguments behind the destruction of life by the Nazis and the use of human embryos (for stem cell research) are scary," he told hundreds of worshippers at a morning Mass. "There are real human lives that need our support and protection."
The petition seeks to put a measure on the 2006 ballot that would amend the state constitution to state that stem cell research, therapies and cures allowed under federal law also are permitted in Missouri. The measure would prohibit human cloning, defined as the effort to create a baby by implanting an embryo that wasn't fertilized by sperm.
Supporters argue stem cells may have the potential to cure spinal cord injuries, Parkinson's, Alzheimer's and other life-threatening diseases.
Opponents contend the use of embryonic stem cells involves creating human life to destroy it.
"Human embryos are not potential human beings. Human embryos are human beings with potential," John Weaver, deacon of Sacred Heart Catholic Church in Columbia, told worshippers Sunday.
Donn Rubin, chairman of the petition coalition, Missouri Coalition for Lifesaving Cures, said surveys show most Missouri Catholics support stem cell research. Republican Gov. Matt Blunt and former U.S. Sen. John Danforth, an ordained Episcopal priest and former U.N. ambassador, are among the measure's supporters.
The petition drive must have about 145,000 valid signatures by May 9 to secure a spot on the November 2006 ballot.
A constitutional amendment would require a simple majority of voters to be enacted and would supersede any state laws.