As a candidate for the Republican nomination for president, Donald Trump refused to release his tax returns.

This was strange behavior in a candidate for national office. Trump’s excuse at the time was that he was unable to share this necessary information with the voting public because his returns were under audit. That’s irrelevant — being under audit has no bearing on whether or not one can make this information public.

Trump’s unexplained evasiveness thus provoked speculation that he was hiding something potentially embarrassing. Perhaps his tax returns showed that he had exploited some corrupt 1-percenter loopholes to avoid paying any taxes at all. Or maybe they showed that his fortune still hadn’t recovered from his bankruptcies in the 1990s and that he wasn’t really the billionaire he claimed to be.

It was impossible to know, because Trump continued stonewalling, offering a shifting series of non-sequitur excuses for his refusal to let citizens see what they needed to see.

More than a year after announcing his candidacy, Trump accepted the Republican nomination at the party’s convention in Cleveland. And he was still hiding his tax returns.

Four months later, despite losing the popular vote by nearly 3 million, Trump narrowly won the electoral college and was elected the 45th president of the United States. And he was still hiding his tax returns.

As a candidate Trump repeatedly promised that he would release his tax returns “at the appropriate time.” In May of 2016 he said “I’ll release. Hopefully before the election I’ll release.” But he instead kept them hidden all through the campaign.

Donald J. Trump was sworn in as president last week, still hiding that information. That likely meant that the moment he took his oath to uphold the Constitution he was in violation of that same Constitution — forsworn in the act of swearing in. Whatever it is that Trump is hiding, it has to be bigger than just potential embarrassment. It has to be worse than that. No one would voluntarily create a constitutional crisis over something that was merely embarrassing.

This is all even more astonishing when we consider how utterly shameless Donald Trump is about so many things that normal, reasonable people regard as disgraceful. This is a man who brags about sexual assault and marital infidelity, who seems proud of his fondness for torture and of his contempt for the weak. Realizing the extent of the horrible things he has no problem regularly revealing about himself makes it even more disturbing to imagine what sort of deeds he would try to hide.

So what is Trump hiding? What is it that is so big and so bad that he’s had to work so hard to hide it for the past 589 days?

We don’t know. We can only guess, based on his sheer determination to keep it hidden. Whatever it is, it has to be yuge.