With Donald J. Trump’s campaign engulfed in crisis, the second presidential debate promised a clash of grand proportions: a decisive, even cataclysmic showdown between one candidate on his heels and the other, Hillary Clinton, emerging as a strong front-runner.

The confrontation did not entirely live up to that billing, but Mrs. Clinton and Mr. Trump vented their vast differences over a revealing hour and a half. Here are some of our key takeaways:

Clinton endured the nuclear attack

Mr. Trump had long threatened to go after Mrs. Clinton for her husband’s infidelities, and had often accused her of enabling Bill Clinton’s transgressions — but he had never said these things to Mrs. Clinton’s face.

On Sunday night, he crossed that line. Claiming that Mrs. Clinton had intimidated women who accused her husband of assault, Mr. Trump told her she “should be ashamed of herself.”

But in the end, a charge long seen as the most incendiary Mr. Trump could offer echoed for only a few minutes. Mrs. Clinton opted not to counterattack, taking the lower-risk course of saying that much of Mr. Trump’s tirade had been false, and quoting Michelle Obama’s case for avoiding retaliation in kind: “When they go low, you go high.”