Byron York explains:

Manhattan District Attorney Alvin Bragg has charged Trump with falsifying bookkeeping records of a nondisclosure payment in order to commit or conceal another crime, Bragg still hasn’t revealed what that other crime is. It’s really the key to the whole case. Without the other crime, there would be no charges against Trump in this matter. The fact that we — and that includes the defendant — still don’t know what the other crime is is one of the great injustices of a felony prosecution that never should have happened...[Bragg's] theory is that if Michael Cohen paid Daniels $130,000 in the fall of 2016 to keep her from going public with her story that she and Trump had a sexual encounter and then Trump repaid Cohen in 2017, then that was a campaign contribution and should have been reported to the FEC. The payments were made “for the purpose of influencing any election,” the theory continues, and the Trump campaign should have filed a document with the FEC listing among its campaign contributions and expenditures that it received and spent $130,000 for “hush money.”

If you think that sounds a little odd for an FEC disclosure, you’re right. That’s where one of the critical witnesses to be called by the Trump defense comes in. Bradley Smith is a former chairman of the FEC, and on many occasions, including long before Trump, he has argued that there are all sorts of things a candidate can spend money on that are not legally classifiable as “for the purpose of influencing any election.” ... Smith, having headed the FEC, has many examples from the commission’s enforcement of federal election law that illustrate his point. He knows what he is talking about, and it seems clear that his expert opinion is that paying off Daniels, no matter what one might think of it, is not a campaign expenditure or donation that FECA requires a candidate to disclose. The Trump defense plans to call Smith as a witness. Not because he has any personal knowledge of the Trump transaction but because he understands, and has enforced, the campaign law that Bragg’s prosecutors appear to be planning to use against Trump. But Merchan has forbidden Smith from testifying about most of the issues involved in the case.