SAN FRANCISCO — Last fall, a group of pioneering scientists, venture capitalists and entrepreneurs sold Californians on the ultimate startup, one with shoot-for-the-moon ambitions. The men and women pitched the state's residents on a new science that they said might one day lead to cures for humankind's worst diseases. "Save Lives with Stem Cells!" campaign posters urged. Today, however, a little more than three months after state voters approved a measure allocating $3 billion in public funds for stem cell and related research, organizers are struggling with more down-to-earth concerns. The initiative has been tainted by accusations that those who...