Memo to poll-obsessed Democrats: Don’t freak out. Hillary Clinton remains a strong favorite to defeat Donald Trump in November. Mr. Trump’s poll numbers rose after he secured his party’s nomination, and hers will when she does. Mrs. Clinton enjoys structural advantages in demography, party registration and the Electoral College. And reading beyond the headline numbers of recent surveys, it isn’t hard to find evidence of the strengths on which she can build a successful election campaign.

As recently as six months ago, it...