The hugely expensive U.S. attempt at nation-building in Afghanistan has had only limited success and may not survive an American withdrawal, according to the findings of a two-year congressional investigation to be released Wednesday.

The report calls on the administration to rethink urgently its assistance programs as President Obama prepares to begin drawing down the number of U.S. troops in Afghanistan this summer.

The report, prepared by the Senate Foreign Relations Committee’s Democratic majority staff, comes as Congress and the American public have grown increasingly restive about the human and economic cost of the decade-long war and reflects growing concerns about Obama’s war strategy even among supporters within his party.

The report describes the use of aid money to stabilize areas the military has cleared of Taliban fighters — a key component of the administration’s counterinsurgency strategy — as a short-term fix that provides politically pleasing results. But it says that the enormous cash flows can overwhelm and distort local culture and economies, and that there is little evidence the positive results are sustainable.

One example cited in the report is the Performance-Based Governors Fund, which is authorized to distribute up to $100,000 a month in U.S. funds to individual provincial leaders for use on local expenses and development projects. In some provinces, it says, “this amount represents a tidal wave of funding” that local officials are incapable of “spending wisely.”

Because oversight is scanty, the report says, the fund encourages corruption. Although the U.S. plan is for the Afghan government to eventually take over this and other programs, it has neither the management capacity nor the funds to do so.

The report also warns that the Afghan economy could slide into a depression with the inevitable decline of the foreign military and development spending that now provides 97 percent of the country’s gross domestic product.

The “single most important step” the Obama administration could take, the report says, is to stop paying Afghans “inflated salaries” — often 10 or more times as much as the going rate — to work for foreign governments and contractors. Such practices, it says, have “drawn otherwise qualified civil servants away from the Afghan government and created a culture of aid dependency.”

Even when U.S. development experts determine that a proposed project “lacks achievable goals and needs to be scaled back,” the U.S. military often takes it over and funds it anyway, the report says.

It also cites excessive use and poor oversight of contractors. Although the report provides some examples of successful projects, it is critical overall of what one senior committee aide called the U.S. focus on a rapid “burn rate” of available funding as a key metric for success. The aide spoke on the condition of anonymity to discuss the report before its release.

Debate has begun within the White House and in Congress over how quickly to begin withdrawing the 100,000 U.S. troops in Afghanistan, with senior Defense Department figures cautioning against a precipitous drawdown this summer. Defense Secretary Robert M. Gates has called for a “modest” decrease that would avoid jeopardizing recent combat gains.

Sen. Carl Levin (D-Mich.), chairman of the Armed Services Committee, told reporters Tuesday that he would like to see a minimum of 15,000 U.S. troops withdrawn by the end of the year. Sen. John McCain (Ariz.), the committee’s ranking Republican, was quoted in the Financial Times as saying that he thought the number should be no more than 3,000.

But an increasing number of lawmakers on both sides have called for a more wholesale reconsideration of Obama’s strategy in Afghanistan, saying that the war’s cost cannot be sustained at a time of domestic economic hardship. They point as well to changing realities on the ground, including signs of growing extremist violence in Pakistan and the killing last month of al-Qaeda leader Osama bin Laden.

“I’m personally for changing the military strategy to some degree,” Sen. John F. Kerry (D-Mass.), chairman of the committee, said in an interview. Because the military and civilian components are tightly intertwined in Afghanistan, Kerry said, both have to be considered at the same time.

“We’ve created a . . . wartime economy” that is a “huge distortion” of Afghanistan’s revenue production, he said. “It’s very dangerous, and we have to get a handle on it rapidly.”

Kerry said that the committee’s report was not “a gotcha” but that it was intended to help the administration “think through and analyze” how to proceed. The report was distributed Tuesday to Democratic committee members and to Sen. Richard G. Lugar (Ind.), the ranking Republican.

The administration has requested $3.2 billion for Afghanistan reconstruction projects in the coming fiscal year. The report argued that the foreign aid program must continue because “the goal should be to reduce some of the political pressure to spend money quickly, especially when the conditions are not right.”

All U.S. development projects in Afghanistan should be reexamined, it adds, to determine whether they are “necessary, achievable, and sustainable.”

The report recommends multi-year congressional funding for the aid program that would plan ahead for the increased civilian responsibilities as the number of troops decreases and calls for “a simple rule: donors should not implement projects if Afghans cannot sustain them.”

Last week, the bipartisan Commission on Wartime Contracting in Iraq and Afghanistan said in a separate report that billions of dollars in U.S.-funded reconstruction projects in both countries could fall into disrepair over the next few years because of inadequate planning to pay for their ongoing operations and maintenance. That report warned that “the United States faces new waves of waste in Iraq and Afghanistan.”

Foreign aid expenditures by the State Department and the U.S. Agency for International Development in Afghanistan, about $320 million a month, pale beside the overall $10 billion monthly price tag for U.S. military operations. But Afghanistan is the biggest recipient of U.S. aid, with nearly $19 billion spent from 2002 to 2010. Much of that money has been expended in the past two years, most of it in war zones in the south and east of the country as part of the counterinsurgency strategy adopted by Obama just months after he took office.

The strategy, devised by Gen. David H. Petraeus, the commander of U.S. and coalition forces in Afghanistan, calls for pouring U.S. development aid into areas that the military has cleared of Taliban fighters to persuade the population to support the Afghan government.

But evidence of successful aid programs based on “counterinsurgency theories” is limited, the Senate committee report says. “Some research suggests the opposite, and development best practices question the efficacy of using aid as a stabilization tool over the long run.”

“The administration is understandably anxious for immediate results to demonstrate to Afghans and Americans alike that we are making progress,” the report says. “However, insecurity, abject poverty, weak indigenous capacity, and widespread corruption create challenges for spending money.”

High turnover among U.S. civilians working in Afghanistan, estimated at 85 percent a year, along with “pressure from the military, imbalances between military and civilian resources, unpredictable funding levels from Congress, and changing political timelines, have further complicated efforts,” it says.

The report is gently but unmistakably critical of the “whole of government” approach implemented by Richard C. Holbrooke, who served as Obama’s special representative for the region until his death in December. Control of all civilian operations on the ground were shifted to the State Department from the USAID, the traditional manager of development assistance.

“This new approach,” the report says, “created new levels of bureaucracy, diminished USAID’s voice at the table, and put decision-making on development issues in the hands of diplomats instead of development experts.”