Donald Trump paying money to Stormy Daniels to keep her quiet is perfectly legal.
Donald Trump paying money to Stormy Daniels to keep her quiet because her story could ruin his election chances is also perfectly legal, as long as it's declared as an election expense.
The case is that Donald Trump paid money to Stormy Daniels to keep her quiet because her story could ruin his election chances and he didn't declare it as an election expense.
While I would love to see a conviction, it's not the strongest case, and it would not surprise me if he's found not guilty.