Honestly it doesn't even matter that much that it was to further his campaign. He made a hush-money payment, he disguised it as a business expense. Business fraud, tax fraud. If I remodeled my house, had my company pay for it, and deducted it as a business expense, I'd be on trial too.
@ScottSoCal That was the first charge. To make it a felony, they proved that the false accounting was done in furtherance of the crime of defrauding voters to influence an election.