Transportation Secretary Norman Mineta, the only Democrat in President Bush's Cabinet, will step down next month.

Mineta's resignation from the Transportation Department is effective July 7, White House press secretary Tony Snow said Friday.

Mineta, 74, was plagued with back problems during his tenure as transportation secretary and spent months working from home and the hospital. But he has since recovered, and Snow offered no explanation for why he has decided to leave now.

Click here to read Mineta's resignation letter to Bush.

Asked why Mineta resigned, Snow said: "Because he wanted to."

"He was not being pushed out," he said. "As a matter of fact, the president and the vice president and others were happy with him. He put in five and half years — that's enough time."

Snow paid tribute to Mineta's long history in public life: his service in the Army, his elections to local positions in California, his 20 years representing California in the U.S. House, and his tours in two Cabinet positions, as Commerce secretary under former President Clinton and now under Bush.

Snow credited Mineta with cutting regulations and red tape to liberalize the commercial aviation market, establishing the Transportation Security Administration, helping to shape the highway bill and injecting "sound economic principles" into the nation's passenger rail system.

He offered no other comment about Mineta's departure, which comes weeks after a shake-up in Bush's top staff and Cabinet had appeared complete.

Mineta had only been expected to stay in the post until major highway funding legislation became law. Congress failed to pass a six-year, $300 billion bill to pay for highway and mass transit programs when the last spending plan expired on Sept. 30, 2003. The previous level of spending was continued until the end of May.