Members of Congress friendly to the Boeing Co. spent three months fulminating against the Air Force’s award of a multibillion-dollar air tanker contract to the European Aeronautic Defense and Space Co. and Northrop Grumman Corp. — but Boeing had more success by appealing the contract award to Congress’ own Government Accountability Office. Two weeks ago, the GAO upheld Boeing’s protest, declaring that the Air Force had bungled the bidding and recommending a do-over.

That decision highlighted the broad powers that Congress over the past 80 years has delegated to the GAO to audit government programs and investigate their management and effectiveness — the stream of reports and committee testimony Washington is most familiar with — and also to arbitrate protests of government contract bids and generally act as a legal adviser to lawmakers and executive agencies on how federal money is spent.

The GAO was established in 1921 to counterbalance the expanding power of the president over the budget process and has periodically been expanded with the decades. For instance, it was given authority over bid protests, like the Boeing case, in 1984.

A separate arm of the agency, run by the general counsel and staffed by about 150 lawyers, is responsible for formal legal opinions, as well as legal advice for the GAO’s hundreds of auditors and congressional committees.

Indeed, the general counsel’s office plays an important role in advising agencies on how they can use the money appropriated to them by Congress and issues three dozen or so formal decisions every year on the subject.

In a recent case, for instance, the GAO advised the Nuclear Regulatory Commission that — should it ever mistakenly disclose to the public personally identifiable information about its employees — it was permissible to spend taxpayer dollars on a credit-monitoring service for the aggrieved workers. The GAO also told the National Telecommunications and Information Administration it was okay to buy gift cards with federal dollars as an incentive for people to return a survey about the agency’s television converter box coupon program.

When a company has bid for a federal contract and loses and thinks the process was unfair, it can protest the decision to the GAO, as Boeing did this spring. Because millions of dollars are often at stake in such contracts, these protests have become common.

The 30 lawyers in the General Counsel’s procurement law division receive nearly 1,400 bid protests a year and deny about three-quarters of them out of hand, says Michael Golden, the associate general counsel in charge of the division. In order to have its protest formally reviewed, the company protesting a contract award must have evidence the winner might not have been qualified, that the company making the bid either misrepresented its capabilities or the time it would take to perform the contract, or that the contracting office acted unfairly.

In Boeing’s case, the GAO agreed with the company that the Air Force made such “prejudicial errors” in evaluating bids that its award to EADS and Northrop Grumman should be called into question.

Of the 300 to 350 bid protests accepted for a formal review each year, more than three-quarters are denied, like a technology firm’s claim last year that the winner of an Air Force contract for advanced engine research had unfair access to information that had not been made public. The GAO denied the protest with the comment that “substantial facts and hard evidence are necessary to establish a conflict; mere inference or suspicion of an actual or apparent conflict is not enough.”

In the 60 to 90 cases each year where the GAO upholds a protest, agency officials who awarded the contract have 60 days to respond to the recommendations. “If they choose not to comply, which happens very rarely,” Golden says, “We are required to write a report up to the Congress, to the four committees of jurisdiction and advise them of the non-compliance, and then it’s up to Congress to decide what to do.”