The House and Senate are fiercely protective of their own prerogatives, not least when they are seeking materials from the executive branch. They take their investigations seriously (even if their principal or sole motivation is political). They do not like to be thwarted. For its part, the executive branch is deeply suspicious of investigations, thinking that they are efforts to make political hay. Its officials do not love to hand over documents. They are fiercely protective of their own deliberative processes, and that is true whether the president is Republican or Democratic.
