Mr Cohen says Mr Trump asked him to make hush-money payments—
something that is not illegal for ordinary citizens,
but counts as an undeclared donation when done on behalf of a political candidate, as Mr Trump was at the time.
After all, America treats breaches of campaign-finance law much more like speeding tickets than burglary:
they are often the result of filling in a form wrongly, or incorrectly accounting for campaign spending.
There are good reasons for this indulgent approach.
When voters elect someone who has bent the rules, it sets up a conflict between the courts and the electorate that is hard to resolve cleanly.
Mr Trump does not stand accused of getting his paperwork wrong, however, but of paying bribes to scotch a damaging story.
That is a far more serious offence,
and one that was enough to end the career of John Edwards, an aspirant Democratic presidential candidate, when he was caught doing something similar in 2008.
There is no way of knowing if Mr Trump would still have won had the story come out.
Even so, the possibility that he might not have done raises questions about his legitimacy, not just his observance of campaign-finance laws.
What of the convention, which has been in place since the Nixon era, that the Justice Department will not indict a sitting president?
Again, there are good reasons for this.
As with breaches of campaign-finance law, such an indictment would set up a conflict between the bureaucracy and the president’s democratic mandate that has no happy ending.
The convention would doubtless be void if there were credible evidence that a sitting president had, say, committed murder.
But the payment of hush money to avoid an inconvenient story about an extramarital affair falls a long way short of that.
The authors of the constitution wanted to allow the president to get on with his job without unnecessary distractions.
But, fresh from a war against King George III, they were very clear that the presidency should not be an elected monarchy.
If a president does it, that does not make it legal.
The constitutional problem that America is heading towards is that the Justice Department’s protocol not to prosecute sitting presidents dates from another age, when a president could be expected to resign with a modicum of honour before any charges were drawn up, as Nixon did.
That norm no longer applies.
The unwritten convention now says in effect that, if his skin is thick enough, a president is indeed above the law.