They're talking like the Trump administration is on shaky ground here. I don't think there is any basis in law where Congress can demand the tax return of anybody without criminal chargers or subpoena.
Trumps gotta start fighting back much harder. I'm sure there is boatloads of dirt on Democrats ... enough to make them pause to think.
That or start arresting people involved in interfering with the executive branch.
OLC Opinion: Congressional Committee’s Request for the President’s Tax Returns
https://www.freerepublic.com/focus/f-news/3756936/posts