President Obama left office with high approval ratings, particularly among Democrats, who still think very well of him. But Joe Biden was the only candidate over the last two night to defend his record and pledge to build on it. I think that his loyalty to Obama will play well with Democratic voters, who have not moved on from the former president to the same extent that Democratic activists and intellectuals have. But I think Biden is leaning too hard on Obama — to make up for the parts of his civil-rights record that are out of step with today’s Democratic party, for example. And that may not play well at all.