The ritual started in earnest last fall in the midst of the biggest humiliation of Barack Obama’s presidency, the failure of the health care website. Anytime he heard a sliver of good news, the president reacted the same way: He knocked on the polished cherry wood table in the Roosevelt Room. It’s a small thing, almost a nervous tic, but Obama’s habit of knocking on wood during Obamacare meetings had become notable, something that close advisers talked and even joked about among themselves. Obama had always projected the aura of a deeply confident man, someone who on the basis of...