*There’s something about that opening of the top job at the Transportation Security Administration that’s difficult to fill. Back in January, Erroll Southers found he couldn’t hand because of politics and dropped out of the running. Now this.

On Friday, retired Army Maj. Gen. Robert Harding took himself out of the running as head of the Transportation Security Administration.

It’s another setback for President Obama in finding a transportation security chief to shore up the nation’s defenses against terrorist threats from the air, road and rail. The White House has called the job the most important unfilled position on Obama’s team.

Questions arose after Harding’s nomination about an Iraq contract his company had. After the government ended the contract early, in 2004, Harding Security Associates claimed more money from termination of the contract than the Defense Department’s inspector general said it was entitled to get. The firm refunded $1.8 million in a 2008 settlement with the Defense Intelligence Agency.

“I feel that the distractions caused by my work as a defense contractor would not be good for this administration nor for the Department of Homeland Security,” Harding said in a late-evening statement released by the White House.