Lawmakers in nearly half the states want to add a requirement for presidential candidates: Show us your tax returns.

The issue has dogged President Donald Trump, who became the first presidential candidate in modern times to refuse to make his returns public. It flared anew this week after MSNBC said it had obtained two pages of Trump’s 2005 federal return, prompting the administration to release the documents preemptively.

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“I think a requirement of revealing one’s tax returns would be regarded as an additional qualification,” said Michael McConnell, a professor at Stanford Law School. “And then there’s the tax law problem, because federal law guarantees the confidentiality of tax returns. And I think that law would pre-empt any state law requiring someone to divulge their returns.”