As Congress prepares to charge AttorneyGeneral Eric Holder with contempt on Thursday for his refusal to deliver documents tied to Operation Fast and Furious, irate Democrats attacked the accuracy and integrity of the Republican probe into the troubled Arizona gun-trafficking investigation, calling it biased and incomplete.

Democrats on Wednesday noted that Darrell Issa (R-Calif.), chairman of the House Oversight and Government Reform Committee, has never in his 18-month probe called the former director of the Bureau of Alcohol, Tobacco, Firearms and Explosives, Kenneth Melson, to testify publicly about the botched operation conceived and executed by ATF agents under his supervision.

Melson has testified only in closed-door and secret hearings, where he told committee members that top Justice Department and White House officials had no knowledge of the flawed "gun-walking" tactics that allowed high-powered weapons to fall into the hands of drug cartels. His full remarks to Congress remain secret.

In a letter to House Speaker John Boehner (R-Ohio), Rep. Elijah Cummings, the ranking Democrat on the Oversight committee, detailed "100 errors, omissions and mischaracterizations" in the contempt report that will be brought to a vote on Thursday. Among those omissions, Cummings said, was the failure to call Melson to testify publicly.

"It appears that the chairman is protecting ATF leaders from our inquiry and refusing to allow committee members to ask direct questions about these operations," Cummings said. He added that he had asked Issa "10 times" to call Melson before the committee.