Correction: An earlier version of this article incorrectly attributed a quote to Christopher Shays, a former Republican House member from Connecticut who served on the bipartisan Commission on Wartime Contracting. It was Dov Zakheim, another commission member who testified at a House hearing last week, who said of the State Department overseeing an estimated 16,000 workers in Iraq: “We’re very, very worried. I don’t know how they’re going to do it.” It was also Zakheim, not Shays, who said he fears a repeat of the 2007 fatal shooting incident in Baghdad that involved Blackwater USA security guards. This version has been corrected.

U.S. soldiers patrol outside Contingency Operating Site Taji, north of Baghdad. (Maya Alleruzzo/ASSOCIATED PRESS)

The State Department is racing against an end-of-year deadline to take over Iraq operations from the U.S. military, throwing together buildings and marshaling contractors in its biggest overseas operation since the effort to rebuild Europe after World War II.

Attention in Washington and Baghdad has centered on the number of U.S. troops that could remain in Iraq. But those forces will be dwarfed by an estimated 16,000 civilians under the American ambassador — the size of an Army division.

The scale of the operation has raised concerns among lawmakers and government watchdogs, who fear that the State Department will be overwhelmed by overseeing so many people, about 80 percent of them contractors. There is a risk, they say, that millions of dollars could go to waste and that bodyguards will lack adequate supervision.

“We’re very, very worried,” Dov Zakheim, a Defense official during the Reagan presidency who served on the Commission on Wartime Contracting, said at a recent House hearing. “I don’t know how they’re going to do it.”

State Department officials say they are working flat-out to finish preparations, adding contracting professionals to prevent fraud and focusing on ensuring the protection of U.S. personnel.

“We’ve spent too much money and lost too many kids’ lives not to do this thing right,” said Thomas Nides, deputy secretary of state.

But officials acknowledge they have never done anything quite like it. “Make no mistake, this is hard,” Nides said.

There are 43,000 U.S. service members in Iraq. Under an agreement negotiated by the George W. Bush administration, they are to leave by the end of 2011.

Iraqi leaders said last week that they want a small contingent of U.S. military trainers to remain, but without immunity from local prosecution, a condition the Obama administration has said it cannot accept. The administration has been planning to keep 3,000 to 5,000 military trainers in the country if the two sides can hammer out an agreement.

The list of responsibilities the State Department will pick up from the military is daunting. It will have to provide security for the roughly 1,750 traditional embassy personnel — diplomats, aid workers, Treasury employees and so on — in a country rocked by daily bombings and assassinations.

To do so, the department is contracting about 5,000 security personnel. They will protect the U.S. Embassy in Baghdad plus two consulates, a pair of support sites at Iraqi airports and three police-training facilities.

The department will also operate its own air service — the 46-aircraft Embassy Air Iraq — and its own hospitals, functions the U.S. military has been performing. About 4,600 contractors, mostly non-American, will provide cooking, cleaning, medical care and other services. Rounding out the civilian presence will be about 4,600 people scattered over 10 or 11 sites, where Iraqis will be instructed on how to use U.S. military equipment their country has purchased.

“This is not what State Department people train for, to run an operation of this size. Ever since 2003, they’ve been heavily reliant on U.S. military support,” said Max Boot, a national security expert at the Council on Foreign Relations.

In its final report, issued in August, the bipartisan Commission on Wartime Contracting said that billions of U.S. taxpayer dollars had been squandered in Iraq and Afghanistan, and that the State Department had not made the necessary reforms in its contracting operation.

“Therefore, significant additional waste — and mission degradation to the point of failure — can be expected as State continues with the daunting task of transition in Iraq,” the commission warned.

State Department officials dispute that conclusion, saying that they have hired dozens of extra contracting personnel and that they have gained experience in managing contractors in Iraq.

Zakheim said he also worries that the State Department’s small security force will be stretched too thin to supervise armed contractors. He told the hearing that he feared a repeat of the 2007 incident in which guards from the security firm then known as Blackwater USA opened fire at a Baghdad traffic circle, killing 17 Iraqi civilians.

Stuart Bowen, the inspector general for Iraq reconstruction, said in an interview that the transition would have other costs. Without military protection, he said, U.S. government workers will have limited reach throughout Iraq. Already, the 1,200 personnel in the consulate in the southern city of Basra cannot move around that region adequately, he said.

“In between this area and Baghdad, there will be a void” of diplomatic coverage, Bowen said.

Nides emphasized that the State Department wasn’t trying to duplicate the military mission.

“That’s not what the Iraqis want. Frankly, that’s not what was agreed to” with the government in Baghdad, Nides said. The department is trying to transition to a diplomatic presence, he said.

Although the Iraq operation will be huge by State Department standards, it will be significantly smaller than the military-led mission, which currently involves 50,000 defense contractors. And State Department officials say their use of contractors is expected to drop sharply over the next three years, as security improves.

Nides noted that the State Department planned to spend less than $6 billion in Iraq in 2012, compared with an outlay of about $50 billion by the military this year.

“That’s a pretty good transition dividend,” he said.

The State Department had originally planned a more ambitious network of consulates and police-training sites, but it scaled back after failing to get enough money from Congress.

Its smaller footprint will be evident in the police-training program, which will be run out of three locations. In contrast, the U.S. military had training programs in all 18 provinces, said Maj. Gen. Jeffrey S. Buchanan, chief spokesman for U.S. forces in Iraq.

“We had a partnership at a much lower level, but I think [State will] bring a very needed expertise at a higher level, a more strategic level,” he said.

The department’s inspector general reported in May that there was a risk that some of the new embassy facilities, such as hospitals and housing, wouldn’t be ready by year’s end.

A State Department official acknowledged that housing construction will probably extend into 2012. But temporary accommodations, at least, will be ready by the end of this year for 10,000 people at the embassy in Baghdad, said the official, who was not authorized to comment on the record. There will be no need, as initially feared, to make people use beds in shifts.

“We will have the basics for everyone,” he said.

Zak reported from Baghdad. Staff writer Greg Jaffe contributed to this report.