In his letter submitted to President Bush Wednesday afternoon and made public Thursday, Johanns said, "After careful thought and difficult deliberation, I am writing to inform you that I have decided to pursue a new opportunity to serve this great Nation."
Two Republican sources said Johanns resigned to make a U.S. Senate run in Nebraska, CNN reported. Johanns was governor of Nebraska before becoming Bush’s Agriculture Secretary.
Johanns said U.S. agriculture is "stronger than ever before in history" under the Bush administration.
"Your presidency has had a profoundly positive impact on the lives of Americans in both rural and urban communities," he said.