AUSTIN – Time is running out for Gov. Rick Perry to decide what to do with a bill that would block state officials from following his executive order requiring schoolgirls to be vaccinated against the virus that causes cervical cancer.

Perry has until midnight Tuesday to sign or veto the bill or it will become law without his signature.

On Monday, Perry spokeswoman Krista Moody said the governor is still weighing his options.

"Governor Perry is thoughtfully reviewing HB1098, and will announce his decision when he feels it is appropriate," Moody said.

Perry, a socially conservative Republican, made national headlines in February when he issued the executive order requiring the human papillomavirus vaccine for sixth-grade girls starting in September 2008.

The vaccine protects girls and women against strains of the sexually transmitted virus that cause most cases of cervical cancer and genital warts. Merck & Co.'s Gardasil is the only HPV vaccine on the market.

The Legislature was outraged that Perry acted without consulting them, and prominent lawmakers promised to do whatever it took to overturn the order.

The bill they sent to the governor last month would block state officials from requiring the shots for four years. At that point, lawmakers could extend the ban or allow health officials to add the shots to the list of vaccinations required for school attendance.

It takes a two-thirds vote of both chambers – 100 votes in the House and 21 votes in the Senate – to override a gubernatorial veto. The legislation passed by well over that margin in both bodies.

Veto overrides are extremely rare, primarily because most major legislation is passed late in the session, when the governor gets extra time to decide what action to take. The last override happened in 1979.

With the odds so heavily stacked against him, Perry has nothing to gain from vetoing the bill, Southern Methodist University political scientist Cal Jillson said. Doing so would only further anger his social conservative base, which loudly complained that the mandate intruded too far into families' lives.

"It's hard to see what the upside for him would be," Jillson said.

The more politically expedient course would be to let the bill become law without his signature and stick by the argument that he prompted an important statewide and national debate, Jillson said.

Perry's comments at a press conference last week suggest he may be leaning in that direction.

"I am quite satisfied that people in this state, people in this country now know that there is a very deadly silent killer that is devastating and that we have a vaccine that can prevent it," he said Wednesday.

Gardasil protects against four strains of the sexually transmitted HPV infection. The U.S. Food and Drug Administration recently approved the vaccine for girls and women ages 9 to 26.

About half of all men and women are infected with HPV at some point in their lives, according to the Centers for Disease Control and Prevention. The agency recommends that girls get the vaccine when they are 11 or 12 so they will have immunity before they become sexually active.

Bills have been introduced in about 20 states to require the vaccine amid some safety concerns and protests from conservatives who say requiring it promotes promiscuity and erodes parents' rights.