The challenge for those who justifiably seek to prevent Mr. Trump from getting away with this is how to respond, without responding in kind. His violation of norms cannot be the basis for a counter-violation resulting in a political race to the bottom. Which brings us to the new California law, just signed by Gov. Gavin Newsom (D), that would deny a spot on the state’s March 2020 primary election ballot to any candidate who fails by late November 2019 to submit five years’ worth of tax returns for public disclosure. In signing the bill, Mr. Newsom said we are living in “extraordinary times” that require the states “to do everything in their power to ensure leaders seeking the highest offices meet minimal standards, and to restore public confidence.”

We share the governor’s belief in transparency but not his confidence that the new law is constitutional, given the Supreme Court’s 1995 ruling — in the admittedly different context of congressional term limits — barring the states from imposing new qualifications for federal office. That precedent at least implies it would be unconstitutional to make ballot eligibility contingent on submitting a tax return, as long as a presidential candidate is a 35-year-old natural-born citizen. The case would be different, and more clearly constitutional, if failure to disclose were punishable by a fine, as campaign finance violations are, but that is not how the new California law reads. The law also would be less objectionable, though in our view still constitutionally suspect, if California’s statute took effect in 2024, rather than now, when it is so clearly aimed at one person.

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As it is, California invites a partisan tit-for-tat. Another state might demand health records, high school report cards, college transcripts or birth certificates — depending on what might be most politically advantageous. Mr. Newsom’s Democratic predecessor, Jerry Brown, cited precisely those concerns in vetoing a previous version of the bill in 2017.

Getting public access to Mr. Trump’s returns is important; so, too, is fighting for that disclosure within the bounds of constitutionality and sound judgment.