Can you support that statement? I don't think that has been established as a fact at all.
In fact, I think the anti-Trump argument is this: It would be illegal to use campaign funds to pay Stormy Daniels. Granted, the money did not come from campaign coffers, so the payment was not illegal in the normal use of the law. However, it is argued that the payment was on behalf of the campaign, so it was (1) essentially the same as an unreported campaign contribution, and (2) if it was a campaign contribution, then it was an illegal use of campaign funds for personal purposes. Which is kind of funny, if you think about it. If the payment was for personal purposes, how could it be a campaign contribution? If his campaign benefitted from the payment, how could it be for personal purposes? But, I think the anti-trumpers will just grab the half of that theory that seems to stick best.
All of this is very interesting, because now private (non-campaign) spending becomes campaign spending under this theory. Every expenditure by a candidate which benefits the candidate in any way becomes both a reportable campaign contribution and a potentially illegal campaign expenditure for non-campaign purposes. Basically, all of their income is automatically campaign money when it suits the prosecutor, and a fair amount of campaign money can become personal when it suits the prosecutor. Nothing like increasing the power of the state over political candidates. I don't see how that can end badly, do you?