Its basically the exact same thing they want from Dan McGahn.
They want the 'internal communications" that the President had with his advisor - in the census case it is his conversations with Wilbur Ross.
The Govt argues that Congress (being its own branch) is not entitled to internal Executive Branch discussions and are not allowed to 'get inside their heads'. If no action was taken or if an action was taken and it was lawful, they aren't entitled to be the thought police.
IF there was no violation of law, Congress is trying to exert oversight they aren't entitled to based on the separation of powers.
This is the exact case that is in front of SCOTUS.
So the D's are just trying to get their outrage machine going.
Scotus has now 4 cases on this topic. There is supposed to be some hearing in the coming weeks. I've been monitoring ScotusBlog.
That's just the way I have internalized it. Happy to be wrong if there are lawfags lurking.