As GOP presidential candidates tussle over the latest issue to split the field — oddly enough, it’s the rather obscure question of whether states should mandate vaccinating girls against a sexually transmitted virus — it’s hard to tell which one ends up looking worst. But our vote goes to Minnesota Rep. Michele Bachmann, whose rumor-mongering rampage against a safe and effective vaccine could discourage parents from protecting their daughters against cancer.

During Monday’s debate, Bachmann and former Sen. Rick Santorum lashed out at Texas Gov. Rick Perry over his 2007 executive order that Texas schoolgirls had to be vaccinated against human papillomavirus (the order was later overturned by the state legislature). Appearing the next day on NBC’s “Today” show, Bachmann claimed Perry had put the lives of girls at risk by mandating “what could potentially be a very dangerous drug.” After the debate, Bachmann said, she was approached by a mother who claimed the HPV vaccine had rendered her daughter mentally retarded. Never mind that there is zero scientific evidence of such a side effect from the HPV vaccine or that five years of wide-scale public use have demonstrated that it prevents deadly cervical cancer at little risk: One unsubstantiated claim from a tearful mom is enough for Bachmann.

Ordinarily, government-ordered vaccination aims at public health threats that are spread through casual contact; HPV, which usually has no symptoms but can lead to cancer, is spread via sex. That makes mandated vaccination particularly irksome for social conservatives, who think the government is encouraging promiscuity by making premarital sex safer. This is such a skewed idea of the reasons teens have sex that we wonder how those holding such views could possibly ever have been teens themselves. According to the National Cancer Institute, HPV vaccination “has the potential to reduce cervical cancer deaths around the world by as much as two-thirds.” Bachmann and her ilk appear less concerned with saving girls’ lives than with protecting their virtue.

Meanwhile, Perry doesn’t exactly emerge from this debate smelling like a rose. The science on the HPV vaccine may be relatively settled now, but it wasn’t in 2007, just a year after federal approval of Merck’s Gardasil vaccine, which is why at the time we urged states to avoid mandates. According to the Washington Post, Perry has received $30,000 in campaign donations from Merck since 2000, and back in 2007 one of his closest confidantes, former chief of staff Mike Toomey, was a lobbyist for Merck while the drug maker was pushing a national campaign to persuade state lawmakers to mandate Gardasil vaccination. Bachmann’s unscientific rhetoric threatens young lives; Perry’s record raises questions about his independence.