Much has been made of two photographs that define George Bush’s presidency. After Sept. 11, he stood in the smoldering ruins of Ground Zero and rallied the nation’s resolve. After Katrina, he flew over the storm’s wreckage in Air Force One, days late, distracted and detached.

The contrast between those images helps explain the president’s plunging popularity. In the latest New York Times/CBS poll, only 36 percent approve of the job he’s doing, and barely 1-in-4 express great confidence in his ability to handle a crisis. Only 29 percent say the country is headed in the right direction.

A year later it’s worth asking exactly why Katrina caused such devastating political fallout. The answer goes far beyond those two photos to the very nature of the presidency itself.

Any American president really holds two jobs at once. One is chief executive, or head of government, the provider of federal goods and services. The second role, chief chaplain or head of state, is charged with embodying and elevating the nation’s spirit.

After Katrina, Bush failed to fulfill both responsibilities.

Start with the head of government role. Natural disasters are always high-stakes events for any executive – mayors and governors, as well as presidents. Ordinary citizens who seldom intersect with government suddenly need help. And that help must be delivered quickly and efficiently, under difficult circumstances, in the full glare of media attention.