I didn't think it was possible but this is an even weaker formulation of the legal argument than the theory advanced in the first letter. Hard to come up with an explanation other than that there is something in those returns they are really desperate to hide from Congress. https://t.co/VmV5BUwl1d— Susan Hennessey (@Susan_Hennessey) April 15, 2019
The legal argument here is not based on case law, because there apparently isn't any case law on this statute (i.e., Congress has never had to defend its use of powers granted by this relevant statute in court). The strongest argument is that Congress is violating some kind of Constitutional principle by asking for the tax returns, because if it is allowed now, it could have been allowed against Dr. King and other civil rights leaders in the '60's (no, I'm not making that up).
To say that the courts will be less than impressed without how this law might have been abused 50 years ago (but wasn't) is an understatement; and if they get past that, they may wonder how the POTUS is equivalent to Dr. King or the SCLC.
The letter argues there is no legitimate legislative purpose served by requesting Trump's tax returns. Delving into Congress' motives invites the same problem as trying to discern the Administration's motives, and despite Trump's clear record of racism, the Supreme Court declined to take that into account in deciding if the final iteration of the Muslim ban it allowed was tainted by it's putative author's racial animus. Because that way lies intrusion into the political process. That door swings both ways, and now it's Congress' turn. Absent a clearly unconstitutional basis for an action, like racial animus (which the Court basically treats as toxic and antithetical to Constitutional order), the Court is usually unwilling to weigh in on a dispute between political actors (i.e., Congress and the Administration). And even then, the animus, obviously, has to be pretty odious. Citing three GOP Senators who don't like what Congress is lawfully asking for is not really evidence of a constitutional infirmity in the request.
Besides, a legitimate legislative purpose for Congress covers a lot of ground, up to and including the conduct of elections in the States (the VRA is neither gone nor forgotten). The "legislative concern" language is not a legal argument, it's a new GOP talking point.
However, "highlighting the word 'shall'" IS a legal argument. It's the most important legal argument for interpretation of this statute. Trying to obfuscate that is trying to yell "Look, over there!" Which is pretty much what that part of the argument amounts to; don't look at the plain wording of the statute (the place courts start in statutory interpretation), look at this shiny distraction that Constitutional issues are involved, and the King wins such arguments because it's good to be King!
They don't speak the quiet part quite that loudly, but they really don't have to, do they?