In the pugilism of this week’s Republican presidential debate, Representative Michele Bachmann seemed to have landed a clean blow against Gov. Rick Perry over an order he issued requiring Texas schoolgirls to be vaccinated against a sexually transmitted virus.

But then in follow-up interviews, Mrs. Bachmann suggested the vaccine was linked to “mental retardation.”

As experts quickly pointed out, there is no evidence whatsoever linking the vaccine to mental retardation — and Mrs. Bachmann ended up shifting the focus off Mr. Perry and on to her long-running penchant for exaggeration.

It is a pattern her current and former aides know well — her tendency to let her passion for an issue overwhelm a sober look at the facts, resulting in indefensible remarks that, in a presidential primary race, are raising questions about her judgment and maturity.