Isn’t it delicious that after trafficking in racism, promoting sexism and using a lie about Barack Obama’s birthplace as a pivot into political relevance, Donald Trump could receive his final death blow from a black woman: the president’s wife?

And isn’t it interesting that after so many years of keeping a studied distance from the ugliness of the political arena, the first lady is throwing herself with such passion into this grotesque campaign?

That says everything about the singular threat that Trump poses, and she’s emerging as the fiercest counter to it: Michelle Obama, octopus slayer. She’s effective because she has never gone looking for a fight — we know that about her. She acts when she has something to defend, and as she made clear in a stirring, searing speech late last week, that’s more than her husband’s legacy, which a Trump victory would decimate. It’s her dignity as a woman. It’s the dignity of all women.

I don’t mean to overstate her impact: Trump was going down before she joined the chorus of condemnation. But her eloquence is sealing the deal. First at the Democratic convention in late July and then in New Hampshire on Thursday, she embodied the nation’s conscience and staked her claim as the most earnest guardian of our most important values.