Trump says he didn't bang Stormy Daniels and then pay her off, but too much has time has passed since he did it so they can't charge him for it.
There are 2 avenues of prosecution of Trump over this matter:
- Campaign finance violations. Payments to Stormy Daniels to suppress negative stories should have been considered a political donation, but they were not declared as such. (This is one of the things that Cohen went to jail for.)
- Business fraud. Payments to Daniels would have been considered a personal expense, but Stubby McBonespurs may have paid from the Trump Organization (and hidden the nature of the payments).
I believe the campaign finance violations would be federal in nature. Currently, the Manhattan DA is investigating the business fraud angle (which might violate both state and federal laws, but the Manhattan DA would likely deal with state laws.)
Now obviously the issue of the statute of limitations comes into play here. However, the fact that they are bothering with the issue at all suggests that they have not yet reached the deadline for that. (Otherwise they wouldn't have bothered with the grand jury at all.)
Complicating the issue is that when Trump was president, the DoJ had a policy of not indicting a sitting president. The problem is, nobody knows how that affects the statute of limitations, since its never been tested in court. (Is the statute of limitations suspended for the 4 years Trump was president because he wouldn't be charged? Or did the clock continue running?)