It was impossible to predict the explosion of relief, enthusiasm, and sense of mission that has greeted Kamala Harris’s presumptive—and now, as of Friday, official—nomination for president. I choose those three descriptors with special care because each represents a critical component of a political phenomenon that has been greater than any of us—most definitely including Donald Trump—could have imagined.

The relief came from Joe Biden’s decision to stand down and not seek reelection. The enthusiasm is for Harris herself—who is, so far anyway, a much more polished and confident candidate than she was in 2019 and early 2020. And the sense of mission derives from what is probably the most important element at work here: the idea that Biden’s departure and Harris’s ascent has presented an opportunity for non-MAGA America to link arms to keep Trump and all his dark designs (you know, the ones he simultaneously praises and denies knowledge of) out of the Oval Office.

The switch to Harris has totally rearranged the chemistry of the race. To take one telling example: Would “weird” have caught on as a meme if Biden were still in the race? First of all, it never would have happened, because Minnesota Governor Tim Walz wouldn’t have had the chance to say it on national television. But even if he had, I’d wager that no, it wouldn’t have taken off. It fed off an online energy, a joyous spirit taken into battle, that just didn’t exist before.

The switch has also opened up new lines of attack against Trump and new positive arguments for Harris and the America she represents. In the former category, most........

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