The Michele Bachmann surge (confirmed most recently by the latest PPP poll) suggests the question is not whether Bachmann is a legitimate contender for the Republican nomination but what it will take to stop her from winning. As I'll explain, I do think Bachmann can be stopped. But the general advance of conservatism within the Republican Party over the last three decades has been a repeated pattern of the unthinkable becoming thinkable, and the trend has sharply accelerated over the last two years. Moderation simply lacks any legitimacy within the GOP. It exists, but -- unlike the Democratic Party, where moderation is a frequent boast -- it's undertaken almost entirely in secret. Since Barack Obama's inauguration, virtually every quarrel within the Republican Party between moderates and maximalist partisans has been resolved in favor of the latter. Bachmann has positioned herself as a mainstream, serious figure who has also outflanked the other as-yet announced candidates. They will have a hard time attacking her without seeming to attack conservatism itself.

So, what could defeat her? One thing could be the entry of another candidate who can match her conservatism while appearing more electable. Rick Perry is the leading candidate here. Paul Ryan would be another.

A second possibility is that Republican insiders could spill the beans on why she so freaks them out. Here, for instance, is Bachmann's former chief of staff:

Having seen the two of them, up close and over a long period of time, it is clear to me that while Tim Pawlenty possesses the judgment, the demeanor, and the readiness to serve as president, Michele Bachmann decidedly does not.

The Bachmann campaign and congressional offices I inherited were wildly out of control. Stacks upon stacks of unopened contributions filled the campaign office while thousands of communications from citizens waited for an answer. If she is unable, or unwilling, to handle the basic duties of a campaign or congressional office, how could she possibly manage the magnitude of the presidency?...

I know Michele Bachmann very well. She is a faithful conservative with great oratory skills, but without any leadership experience or real results from her years in office. She is not prepared to assume the White House in 2013.

More of this -- forthrightly making the case that she's completely unqualified to serve as president -- would help disqualify her among voters who would otherwise be receptive to her ideological arguments.

And then the third possibility is that Bachmann makes more gaffes that make her seem like, well, a flake. Posing as a victim of the sexist media can inflame supporters, but it only takes you so far. Bachmann probably does face a higher standard of scrutiny than her rivals (imagine what would happen if she launched an extended discussion of Iraq policy referring to the country as "Iran" and "Iranians"?) A few more gaffes would play into factor number two.