Democrats and Republicans have been ramping up their critiques of each other over Operation Fast and Furious in the days leading up to U.S. Attorney General Eric Holder's testimony Thursday.

The Justice Department fired the latest salvo today when they responded to a threat by Rep. Darrell Issa to hold Holder in contempt of Congress for responding inadequately to a subpoena.

"Your criticisms of the Department in general, and Assistant Attorney General for the Criminal Division Lanny Breuer, in particular, seem predicated on significant misunderstandings both of the documents we recently produced and of the Department's positions on the issues you raise," Deputy Attorney General James Cole says in the letter (attached).

The exchange was the latest of several beginning when the Justice Department sent congressional investigators a batch of documents Friday night.

In response, Issa's staff put out a press release noting this Associated Press story. It cites the documents released Friday, which show that Breuer suggested an investigation involving a form of gun-walking, the controversial tactic used in Fast and Furious. The documents also show that Monty Wilkinson, then the deputy chief of staff to Holder, was told on Dec. 16 2010, a day and a half after Border Patrol Agent Brian Terry was killed, that guns from an ATF investigation were found at the scene of Terry's murder.

This set off reports in some conservative news outlets that Holder likely knew much earlier than he has admitted about the gun-walking in Fast and Furious.

Then late on Monday, the Democratic members of the House Oversight Committee, which Issa chairs, released their own report on Operation Fast and Furious (attached). The conclusions mentioned in this paragraph raised hackles from Republicans:

"This reports debunks many unsubstantiated conspiracy theories. Contrary to repeated claims by some, the Committee has obtained no evidence that Operation Fast and Furious was a politically motivated operation conceived and directed by high-level Obama Administration political appointees at the Department of Justice. The documents obtained and interviews conducted by the Committee indicate that it was the latest in a series of reckless and fatally flawed operations run by ATF's Phoenix Field Division during both the previous and current administrations."

Sen. Chuck Grassley, the Republican who revealed the gun-walking in Fast and Furious last year, released a disgusted response.

“The idea that senior political appointees have clean hands in these gunwalking scandals doesn’t pass the laugh test, especially considering we’ve seen less than 10 percent of the pages that the Justice Department has provided the Inspector General. They ignored the warning signs and failed to put a stop to it or hold anyone accountable.  Lanny Breuer is a senior political appointee, and he admits to knowing about gunwalking as early as April 2010."

Then Tuesday, in response to Friday's release of documents by the Justice Department, Issa sent Holder a letter (attached) threatening to hold him in contempt of Congress.

"The fact that the Department just produced this document on Friday shows the lengths to which you are willing to go to obstruct our investigation and deceived the public," Issa wrote.

Holder's testimony before Issa's committee is scheduled to begin Thursday at 7 a.m. Arizona time. It can be viewed live at the House Oversight Committee's website.