California lawmakers are preparing for another round of heated talks Wednesday on whether the state should prevent parents from seeking vaccination exemptions for their children because of religious or personal beliefs.

The Senate Education Committee is the second body to consider a bill that supporters say would increase the number of schoolchildren who are vaccinated in the wake of a measles outbreak that started at Disneyland in December. But well-organized opponents have responded with impassioned pleas against the proposal, saying the bill tramples on parental rights.

The topic generated such an acidic debate last week that the proposal's author, Sen. Richard Pan, a Democratic pediatrician from Sacramento, received added security. In addition to threatening messages sent to his office, opponents of the legislation have posted images online comparing Pan to Adolf Hitler.

If the bill passes the Legislature and is signed by the governor, California would join Mississippi and West Virginia as the only states with such strict vaccine rules. Only medical exemptions would be available for children with health problems.

Robert Kennedy Jr., the nephew of President John F. Kennedy and son of former U.S. Attorney General Robert Kennedy, is among those who oppose the bill. At a film screening in Sacramento last week, he described the number of children injured by vaccines as "a holocaust."

Kennedy apologized this week for the comparison, saying he was trying to convey the effects of autism on children and their families. The medical community says such claims have been scientifically disproved.