Abuse of power.
Obstruction of Congress.
I can see how the dems will impeach on abuse of power, but obstruction on Congress is laughable. How many supreme court cases have established executive privilege?
"The supreme court has never addressed executive privilege in the face of a congressional demand for information." (thiis a good read from SCOTUS blog).
The first and ONLY Supreme Court ruling to recognize any type of executive privilege was US vs Nixon. It did so narrowly, acknowledging a limited executive privilege while ruling against Nixon's claims of absolute privilege. This was not a demand of Congress but a demand from a special prosecutor. There's a couple of other cases ruled on by lower courts that sharpened the definition that SCOTUS did not take up. Those decisions outline things a bit better, and absolutely give the White House a basis for which to base their privilege claims.
But here's the problem - They aren't invoking executive privilege. They've invented something else all together. Pat Cipilone's letter to congress said that the administration would simply choose not to participate, would allow no one to testify, and would provide no documents. That is completely out of line with any court precedent.