WASHINGTON — The United States will not be able to protect its diplomats in Iraq adequately if it sticks to the plan to withdraw its last 50,000 troops by December, potentially hindering American efforts to reach out to the Iraqi people, according to a new report by the Senate Foreign Relations Committee.

The report, to be released Tuesday, contends that if the Obama administration leaves only a token contingent of troops behind in an advisory role, as currently planned, “security and political gains could be jeopardized.”

Without thousands of additional soldiers — a prospect that seems untenable, given political pressures in both countries — the report recommends rethinking the American civilian presence, which is projected to number 17,000 diplomats, contractors and others in 15 sites in Iraq.

“The administration may be forced to choose between scaling back the diplomatic mission or accepting a degree of physical risk familiar to military personnel but normally unacceptable for diplomats,” said the report by two committee investigators, a copy of which was provided to The New York Times.

On Tuesday, the two top American officials in Iraq, Ambassador James F. Jeffrey and Gen. Lloyd J. Austin III, the senior American military commander in the country, are scheduled to testify before the Senate about the handover to civilians from soldiers.

While the report says that Mr. Jeffrey and General Austin have built a healthy relationship, it found evidence of bureaucratic squabbles between the Pentagon and the State Department — making it easier, for example, for the military to deliver helicopters to foreign countries than to the State Department.

Among the report’s most startling conclusions is the formidable security detail needed to protect new American consulates in the cities of Basra and Erbil, as well as tiny outposts in Kirkuk and Mosul. Securing the consulates will require 1,400 security personnel for the 120 civilians, the report concludes, while the outlying offices will need a security staff of more than 600 for only 30 staff members.

All this could cost between $25 billion and $30 billion over the next five years, the report estimates, recommending that the administration ask Congress for extra money outside the State Department’s regular budget.

The State Department has been bulking up for months for the postwar era in Iraq, when it will take over the lead role from the Pentagon, training the Iraqi police and overseeing an Office of Security Cooperation, staffed with a few hundred American soldiers who will help Iraqi troops with defensive operations.

The department plans to hire 5,500 private security contractors, roughly double the current number; most will guard the embassy in Baghdad and the four satellite outposts. The security of the installations themselves is adequate, the report says, although it questions whether contractors should take over delicate assignments like bomb disposal and aerial surveillance.

But protecting diplomats as they move around Iraq is a much bigger problem, the report says. The issue is not one of armed bodyguards — the report says less firepower might actually benefit the diplomatic mission — but the loss of intelligence, surveillance and rapid response capability that the military has been providing.

“The satellite sites will only be as effective as their inhabitants’ ability to get off their compounds,” the report says.

The Senate staff report does not estimate how many soldiers will be needed to provide adequate security. Military officers have suggested 5,000 to 10,000. But President Obama and Prime Minister Nuri Kamal al-Maliki of Iraq have each vowed to stick to the December 2011 deadline for a total American troop withdrawal.

Given that reality, the report concludes, “the United States should consider a less ambitious diplomatic presence in Iraq.”