Senate Bill 277 will be heard Thursday at the Capitol.
Sen. Dr. Richard Pan's staff confirmed they have received numerous death threats since the introduction of the bill, which would require most parents to vaccinate their schoolchildren and eliminate personal and religious belief exemptions.
Pan, who introduced the bill in February, was not available for an interview Wednesday. However, his office has added another phone line, presumably because of the high number of inquiries and calls coming in about SB 277.
Meanwhile, the bill is expected to draw a crowd of people Thursday.
Celebrity and TV actress Jenna Elfman Tweeted "Off to Sacramento to make my voice heard to retain parent's rights, against Senate Bill 277! Who's with me?"
Randy Thomasson, president of savecalifornia.com has been a big opponent of the bill but doesn't support threats.
"No one should threaten death. But death to this bill would be very appropriate," he said.
The office of the senate president pro tem would not comment on the threats or any security details.