Michele Bachmann, who’s seen support for her presidential campaign slip badly over the last several weeks, finally thought she’d found a winning issue against Rick Perry, the rival who’s taken so much of her backing. At this week’s debate, the right-wing Minnesotan slammed the Texas governor for providing a human papillomavirus (HPV) vaccine to girls in state schools.

It was a clean hit on an issue that could conceivably resonate with Republican voters. But Bachmann couldn’t leave well enough alone — she soon after claimed that the HPV vaccine can lead to mental retardation in girls, a claim with no foundation in reality. The congresswoman then repeated the claim in two more interviews.

Suddenly, the discussion was no longer over Perry’s vaccine policy, but over Bachmann’s penchant for saying ridiculous things in public.

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. […]

People close to the campaign … spoke of their frustration that Mrs. Bachmann, who entered the race with a reputation for making unsupportable statements on cable television, has not found the discipline to win credibility with major Republican donors and influential referees in the conservative news media.

For anyone who’s paid any attention to Bachmann’s career, those questions are long overdue. When she was little more than a go-to clown — a politician the cable-show bookers could count on to say strange things on national television — concerns about her connection to reality didn’t seem especially important to the political mainstream. But with a national spotlight shining on her presidential campaign, it’s dawning on some folks to stop and say, “Wait, she appears to be a crazy person.”

And the more Bachmann stays in the spotlight, the more people realize she just makes stuff up — all of the time, on a wide variety of issues. There are, of course, competing explanations for this. I’d argue that Bachmann just comes up with bizarre gibberish because she’s unhinged, driven to madness by a twisted ideology. Bachmann supporters insist she’s “unscripted” and driven by passion for the subject matter, leading her to take occasional liberties with reality.

Either way, the explanation is less significant than the conclusion: much of Bachmann’s arguments are simply and objectively nonsensical.

While this isn’t new to many of us, the vaccine story appears to be uniquely damaging, in part because she was perceived as faltering anyway, making the controversy a poorly-timed setback. Jim Dyke, a former communications director for the Republican National Committee unaffiliated with any candidate, said, “This is the nail in the coffin in her campaign.”

That may sound excessive, but just over the last few days, Bachmann’s anti-vaccine comments have come under fire from, among others, Rush Limbaugh, the editorial page of the Wall Street Journal, Bachmann’s former campaign manager, and the American Association of Pediatrics.

It’s not just a tough mistake to recover from; it’s a tough mistake that reinforces the worst fears people had about her.

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Follow Steve on Twitter @stevebenen. Steve Benen is a producer at MSNBC's The Rachel Maddow Show. He was the principal contributor to the Washington Monthly's Political Animal blog from August 2008 until January 2012.