"There is no doubt that the Constitution gives Congress the right and the responsibility to investigate," he wrote. "This is an integral part of our legislative function, one that I know you take very seriously.
"But the Constitution also requires something else: it requires Congress and the executive branch to avoid unnecessary conflict and to seek accommodations that serve both of their interests."
Cummings also said the report finds that Holder has said he would not hand over several categories of documents, even though many of these are blocked by law from being given to Congress.
The letter also notes that Committee Chairman Darrell Issa (R-Calif.) has denied Democratic requests to hear testimony from Ken Melson, the former head of the Bureau of Alcohol, Tobacco, Firearms and Explosives.
"Even though the Attorney General has testified nine times, the Chairman has refused our repeated requests to hold even one hearing with the leaders of ATF," he wrote.
Democrats have also called for former Attorney General Michael Mukasey to testify about the evolution of the gun-walking program as early as 2006, under the Bush administration. Democrats have tried to tie the program to the Bush administration this way, and Cummings wrote that Mukasey was told that despite early failures in the program, the ATF wanted to expand these sorts of operations.