Were he any other person, they certainly would have presented this information to a grand jury for indictment by now. But the Department of Justice’s view is that sitting presidents cannot be indicted because it would undermine the executive branch’s ability to perform its constitutionally assigned role.
This question was last addressed by the Office of Legal Counsel during the Bill Clinton administration. However, it actually extends back to the Nixon administration, which first determined that sitting presidents could not be indicted. Changing that view now would validate all of Trump’s claims of being persecuted by the federal bureaucracy. This doesn’t mean that Trump is safe; given the five-year statute of limitations on campaign finance crimes, it means only that Trump is safe for now.
Trump also has several potential avenues to attack an indictment as described. Obviously, he can try and establish that Cohen didn’t act at Trump’s direction. He could also show that the payments were not for the purpose of influencing the election, but for a different reason (protecting first lady Melania Trump’s feelings, for example).If true, that would mean that, as far as Trump knew, he wasn’t directing campaign contributions. But even a defense like this opens Trump up to additional legal jeopardy if he lies to investigators.