Washington (CNN) -- President Obama has tapped a former Army general to lead the Transportation Security Administration, sources have told CNN.

Obama plans to nominate Robert A. Harding, a retired major general with 33 years in the Army, to become the TSA administrator, sources said. Department of Homeland Security Secretary Janet Napolitano will announce the nomination Monday with Harding by her side, according to one administration official.

"The TSA administrator is the most important unfilled post in the Obama administration," one administration official said. "Mr. Harding has the experience and perspective to make a real difference in carrying out the mission of the agency."

"If there was ever a nominee that warranted expedited, but detailed, consideration in the Senate, this is it," the official said.

In September, Obama nominated Erroll Southers, a Los Angeles airport police department official, to head the agency. But Republican Sen. Jim DeMint, R-South Carolina, spearheaded GOP efforts to block the nomination based on concerns Southers would unionize airport screeners.

Southers withdrew his nomination in January after lawmakers questioned his changing explanation about a personnel action taken against him decades ago.

Harding would be the TSA's first African-American administrator. Southers also is black.

Harding has served as CEO of Harding Security Associates, a defense and intelligence government contractor that he founded in 2003 and sold in July 2009.

From 1996 to 2000, Harding was director for operations at the Defense Intelligence Agency, where he was the Defense Department's senior human intelligence officer.

Before that, he was director for intelligence for the Army's Southern Command.

The TSA was created in the months after the Sept. 11, 2001, attacks and soon took over security at the nation's airports, including screening commercial airline passengers and luggage.

CNN's Jeanne Meserve and Mike Ahlers contributed to this report.