At first I assumed this was an anti-Bush bumper sticker.

From Danelle:

Seen in North Texas

Bumper Stickers - North Texas - Cowboy White House left
Bumper Stickers - North Texas - Cowboy White House right

But Danelle sent this link to the song Cowboy in The White House, and an article from 2003 about the song:

The song is a powerful message of a singular man in our White House that needs the support of Americans. We live in a day of terrorism and violence. We are in the midst of a war like no other in history. The song calls us to support our President, our leader and a true Cowboy.

The song is a powerful message of a singular man in our White House that needs the support of Americans. We live in a day of terrorism and violence. We are in the midst of a war like no other in history. The song calls us to support our President, our leader and a true Cowboy