In the latest grim tiding of the public mood, merely 42% think the American dream that “if you work hard, you’ll get ahead” remains true, down from 53% in 2012 and 50% in 2010. According to the Public Religion Research Institute poll last week, the steepest declines in belief in the last two years were among people under age 30 (down 16 percentage points), women (14 points) and Democrats (17).

In other words, the most disillusioned belong to the coalition that elected President Obama. But before giving up on upward mobility,...