Let me explain why this issue is so important to me — and, indeed, much bigger than Mr. Trump and the current campaign for the presidency.

For one, it’s not really about his tax records per se. It’s about the American public’s ability to see other candidates’ returns. We have a long precedent in which every major-party presidential candidate since I was a child has released his returns. Break it now, and it stays broken.

The presidency is the most powerful political position on earth, and the idea of enabling the voter the chance to see how a candidate has handled his or her finances is a central part of making sure the right person gets the job. There is a reason a banker wants to see tax returns in determining whether you are eligible for a mortgage. You may talk a good game; tax returns don’t. Mr. Trump knows all this, which is why his team had his running mate, Gov. Mike Pence of Indiana, disclose his tax returns — again, an accepted and expected practice in vetting potential vice-presidential candidates.

In fact, the real issue is not even about presidential tax returns. Rather, it’s about the hundreds of down-ballot races, in states and localities, and the transparency voters deserve here, too. I ran twice for governor of South Carolina, and I released my tax returns both times. To be frank, it felt a bit like a colonoscopy: I didn’t like it, but it was our tradition in South Carolina. The power of staying true to the precedent that had been set prevailed. If presidential candidates won’t release their tax returns, you can expect the same in the states. If a presidential nominee doesn’t do it, why should a candidate for governor?

And it matters in ways that aren’t immediately obvious. I once participated in a debate in Congress on raising the president’s salary. As it turned out, the debate wasn’t really about the president’s earnings in office — modern presidents wind up anything but poor — but instead about the pay scale for federal judges: Their pay was ultimately capped at a fixed range below the president’s, so for judges’ pay to rise, the president’s had to also. It was an important reminder: As with so many things in our country, the standards we set for the president determine what political standards we set for the rest of the country.