California Gov. Gavin Newsom has signed a bill requiring Presidential candidates (as well as gubernatorial candidates) to supply their tax returns if they want appear on the primary ballot. There is no question the Trump campaign or their allies, and possibly others, will file a constitutional challenge to this law in federal court. The key argument would be that this law imposes additional qualifications on presidential candidates in violation of Article II of the Constitution. I think such a challenge has a pretty good chance of succeeding, although nothing is certain.
Back in 2017 in a Politico piece, I explained the main constitutional argument about qualifications, and how the state may try to get around it:
The idea of using ballot access to force politicians to do something they don’t want to do is not new—nor is the fight over the move’s constitutionality. The Supreme Court’s 1995 ruling in U.S. Term Limits v. Thornton held that Arkansas could not deny ballot access to congressional candidates who had served more than three terms or to Senate candidates who served more than two terms—measures the state had enacted to create congressional term limits. The Court held that the Constitution set the exclusive qualifications for running for federal office (including age and citizenship requirements), and that allowing individual states to impose addition qualifications “would erode the structure designed by the Framers to form a ‘more perfect Union.’” So while states can set reasonable conditions for presidential candidates to get on the ballot, such as requiring a certain number of petition signatures to be listed, they cannot go further and set substantive conditions for who can run.
In light of such precedent, what could possibly be the basis for upholding a state law barring ballot access for presidential candidates who decline to release their tax returns? After all, Article II of the Constitution includes exclusive qualifications for the office of the president: The president must be a natural-born citizen who is at least 35 years old and has resided in the United States for at least 14 years.
The answer lies in another part of Article II—the part that received some important attention in Bush v. Gore, the Supreme Court decision that Democrats love to hate. Famously, the 2000 case between Republican nominee George W. Bush and Democratic nominee Al Gore effectively handed the election to Bush when it ended the Florida recount.
I also expect that there will be arguments based upon the First Amendment right of a political party to be able to object to conditions put on a party primary. Already cases hold that political parties can insist on closed primaries when a state wants them open to independent voters.
I expect that a lawsuit will be filed in federal district court soon. In addition to the complaint, there will be a request for a preliminary injunction or other emergency relief so that the issue can be decided, at least on a preliminary basis, in time for the upcoming election.
It would not be surprising for such a case to make its way through the Ninth Circuit and to the Supreme Court within a matter of months.