The case that the IRS has to hand over the tax returns is 100% ironclad, since the statute says “shall” and the request is clearly within legitimate Congressional oversight functions.
Mnuchin is being lawless in refusing to comply. It’s true that one generally has the right to contest claims with which one disagrees legally, and then let the courts decide. But here the grounds for objection are frivolous. I thought it was settled back in Magna Carta days, or certainly before the American Revolution, that the Parliament had oversight powers with regard to the executive branch, and the U.S. Constitution clearly reflects a stronger balance of powers commitment than they had in those days across the Pond.
Mnuchin’s position is apparently that Congress has no oversight powers. This is not within the range of debate about what the U.S. Constitution might mean, whatever one’s interpretive theory. And no one at even the very highest levels of the Administration actually believes it, at least in the sense of agreeing that a Democratic president would be similarly exempt from Congressional oversight.
It’s true that Mnuchin is being ordered from above not to comply. But when you receive a lawless order, your choices are to decline to follow it, or else to resign.
The IRS memo has no authoritative legal force as such, but it shows two things. The first is that the IRS lawyers realized how clearcut and indisputable the legal issue is. (The mention of executive privilege presumably reflects a view that such issues would be outside IRS legal expertise, but of course it’s completely absurd to argue that a president could have executive privilege regarding the tax returns he filed as a private citizen, and also no such claim has been made.)
Second, the fact that the IRS memo apparently played no role in the Administration’s, or Mnuchin’s, consideration of what to do unsurprisingly confirms their lawlessness. In an Administration that viewed legality as at least relevant, the memo would have been considered, and if its analysis was rejected that would have been based on non-risible legal arguments to the contrary. That does not appear to be what happened here.