In a speech on the Senate floor, Grassley said his charges are detailed in two reports sent to the White House, congressional oversight committees and GSA. He said he could not release the reports publicly due to proprietary and legal issues.
GSA did not respond to requests for comment.
Grassley repeated charges that GSA Administrator Lurita Doan engaged in back-channel communications with Sun Microsystems while GSA contract officers were in heated negotiations with the company over renewal of a long-term deal.
Sun faced allegations it defrauded the government by overcharging during the contract's initial years.
Documents cited by Grassley and the House Oversight and Government Reform Committee show Doan told her staff that losing the Sun contract would hurt GSA financially.
The agency, which negotiates government-wide contracts with vendors, gets most of its budget from fees other agencies pay to buy from GSA schedules. Critics say that arrangement causes GSA to value contract volume over price.
After Doan expressed concerns, officials under her helped remove two contract officers who opposed renewing Sun's contract from the negotiations.
The third officer quickly made concessions critics say amounted to a bad deal for the government, a charge GSA strongly denies. Citing pressure from Grassley, Sun last month canceled the contract.
Grassley's charges culminate nearly a year of criticism he has leveled at Doan. His staff began questioning agency officials after reports last year that she tried to limit the budget of GSA Inspector General Brian Miller during a dispute over contract audits.
Witnesses have said Doan once privately compared IG employees to terrorists. Publicly, she has said they often intimidated agency employees and contractors through aggressive tactics.
Sun has made similar charges. In an Aug. 9 response to a Justice Department lawsuit, Sun said IG officials who negotiated with the company showed "animus."
An IG official in 2006 said Sun "would never receive another government contract" if its negotiators did not accept GSA's terms, Sun claims.
But Grassley Wednesday said Doan provided little evidence in response to his invitation to substantiate her claims about IG employees. He said just one example was brought to his attention.
"My staff could find no evidence to support those allegations," he said. "Sadly, it appears as if that one specific allegation may have been fabricated to cover intense top-down pressure on the contract officer to award a contract."
The Sun charges are among a series of issues raised against Doan, a President Bush appointee, by Grassley and Democratic legislators.
In June, the independent Office of Special Counsel sent Bush a report recommending Doan be fired for violating the Hatch Act, which bars using federal resources for partisan politics. The White House has not responded.