Swiss voters overwhelmingly approved a law allowing stem cell research Sunday, rejecting a hard-line campaign that compared researchers to the Nazis’ “angel of death,” Dr. Josef Mengele. Some 66.4 percent of those polled — or 1.1 million voters — approved the law passed by the government last December. The law will take effect in March. Opponents had called the referendum to try to overturn the legislation, even though it sets stricter limitations on research than exist elsewhere in Europe. The Swiss bill only allows the use of embryonic stem cells left over from in-vitro fertilization.