But moderate Missouri Republicans have to deal with those pesky party supporters who believe that improving the quality of life, or extending life, isn't "pro-life" if stem cells are destroyed to attain those goals. Because Gov. Matt Blunt defended the proposed amendment, Missouri Right to Life no longer regards Blunt as "pro-life."

"This referendum has the potential to rip our party apart," Representative Kenny Hulshof warned fellow Republicans at a recent statewide convention.

Missouri Republicans who align themselves with the "pro-life" position risk alienating the Missourians (two-thirds, according to a recent poll) who support the amendment. Politicians who depend on the religious right to win elections face a tough choice. Some are trying to remain silent, a strategy of evasion that will be difficult to sustain until November.

Among those caught in the middle is Senator Jim Talent, a Republican who faces a re-election challenge in November from the state auditor, Claire McCaskill, a Democrat. Mr. Talent recently dropped his support of a federal bill to criminalize the cloning of human embryos but has not taken a position on the ballot measure. Ms. McCaskill supports it.

Republicans like Stella Sollars, who opposes embryonic stem cell research, are monitoring Mr. Talent's every utterance on the topic. "That issue of stem cells is the undercover issue that people are going to be voting on," Ms. Sollars said.