What is the Constitutional Ground for the DOJ?

The question is rhetorical; I assume you all know the answer. It is not a Constitutional organ, but rather an old executive office, an outgrowth of the office of the Attorney General that was set up by a 1789 law passed by Congress and signed by a President -- George Washington, no less. 

And there shall also be appointed a meet person, learned in the law, to act as attorney-general for the United States, who shall be sworn or affirmed to a faithful execution of his office; whose duty it shall be to prosecute and conduct all suits in the Supreme Court in which the United States shall be concerned, and to give his advice and opinion upon questions of law when required by the President of the United States, or when requested by the heads of any of the departments, touching any matters that may concern their departments, and shall receive such compensation for his services as shall by law be provided.

Neither the Attorney General nor the DOJ is meant to be independent of either Congress or the President, from whom all their authority is derived. Insofar as the Attorney General is misusing his power, those branches are responsible for him. That means that, like it or not, he answers to them. 

Attorney General Merrick Garland struck a defiant tone Wednesday in defending the Justice Department as independent of the White House and Congress, but Republicans attacked him repeatedly for the handling of high-profile investigations of Hunter Biden and Donald Trump.

"Our job is to uphold the rule of law," Garland told the House Judiciary Committee in an uncharacteristically emotional statement....

Garland reminded lawmakers, according to the prepared remarks, that he represents the American people rather than the president or Congress.

“Our job is not to take orders from the President, from Congress, or from anyone else, about who or what to criminally investigate,” Garland said.  

That is simply not true. Practically, he acts as if he knows it: he is clearly being guided by political imperatives in his handling of cases both high and low profile. The independence he pretends to is unconstitutional and improper. 

Here as elsewhere, I am not suggesting a program of reform but just trying to speak the truth about it. Congress is toothless against the bureaucracy and does not want their power back; the President is a nonentity, and none of his proposed replacements have the necessary virtues either. The DOJ is doing what its leaders please to do, politically: they are indeed functionally independent, exactly as they should never be. The system is broken, and is not capable of fixing itself.