Barack Obama has fallen below 50 percent in the Gallup Poll, an expected but no less significant historical marker.

The public's view of this president ranks him in the lower third of modern American politics. He has fallen below the majority threshold at the fourth fastest rate, of the twelve presidents since World War II.

Obama was nearly number three. But he escaped Ronald Reagan's fall-point by about a week. That Reagan has been here as well, and languished below 50 for two years in his first term, illustrates that Obama's new reality is notable but not determinative. Still, it is indeed bad news.

In legislative terms, a president is only as powerful as he is popular. The metric for that popularity is the public approval rating.

Below 50, the president can...