In Tuesday’s debate against Donald Trump, Kamala Harris emphasized her middle-class background, her admiration for small business owners and her plans for an “opportunity economy.” She promised tax credits for young families and support for first-time homeowners. Those attempts to put meat on the bones of her economic plan will not be enough to win the presidential election.In the latest Times/Siena poll, Ms. Harris trailed Mr. Trump by 13 points on the economy, the issue that matters most to voters. It’s why she’s merely tied with Mr. Trump in the race overall, not crushing him.Optimistic words and some admirable proposals...