An executive with a Dubai-owned company withdrew his nomination as head of the agency that oversees ports in a letter to President Bush on Monday.

Bush in January nominated David Sanborn, DP World's director of operations for Europe and Latin America, to head the Maritime Administration.

Two Democratic senators objected.

Shortly after the nomination, DP World's planned purchase of a company that runs six major U.S. port facilities became the center of a roiling political controversy.

DP World did complete its purchase of London-based Peninsular and Oriental Steam Navigation Co., but DP World since agreed to sell its U.S. port operations to a U.S. company within six months.

Sanborn, a graduate of the U.S. Merchant Marine Academy and a retired Naval officer, said in his letter that the day he was nominated was the proudest of his life.

"While I believe my background makes me one of the most qualified people there is for this position, the convergence of a number of factors bring me to the conclusion that I cannot effectively serve my country, you, and the U.S. maritime industry," Sanborn wrote.

Former presidential candidate John Kerry, D-Mass., and Sen. Bill Nelson, D-Fla., put holds on Sanborn's nomination, saying they needed to know more about his role in the process that allowed DP World to purchase the port operations in the first place.

"I hope the withdrawal of this nomination means the administration is finally waking up to the homeland security problems with our ports, not just saving face after the merciful end of the Dubai debacle," said Kerry.

The Maritime Administration, or MARAD, keeps data on port traffic, disposes of obsolete ships, runs the Merchant Marine Academy and works with the Pentagon on a program to identify U.S.-flagged ships that can transport military cargo.