After the Ethics Reform Act of 1989 they can't just pocket the money. But they can lend their campaign as much as they want and then pay back the loans at the end. And they can even keep on collecting contributions after they end the campaign to pay of those debts.
They can also keep the dough for a future run, give it to other candidates or PACs (within limits), give it to charity, or return it.
But Trump probably won't have any money leftover since he is spending it as fast as he collects it by paying his companies and family lavishly. Somebody looked into his campaign spending and some of the shell companies are pretty much non-existent.
Apparently really poor management of campaign funds is not quite criminal, that being Trump Standard Operating Procedure.
