In the span of one week, we learned that the Office of the U.S. Attorney for the Southern District of New York had both secured a guilty plea from Trump Organization lawyer Michael Cohen and offered an immunity deal to the company's chief financial officer, Allen Weisselberg. President Donald Trump should be worried. Once the Southern District gets its jaws onto a string of crimes, it doesn't let go.

Weisselberg, as part of his deal, will likely be required to provide information on all criminal activity he knows about. That spells potential disaster for Trump personally, and major problems for his presidency. That's apart from any potential state-level criminal investigation by the New York district attorney's office.

Trump is now facing a two-front war against the Justice Department. The team led by special counsel Robert Mueller is supposed to focus on Russian interference in the 2016 election. But the Southern District can investigate any aspect of Trump's behavior that took place in its jurisdiction, at any time.

And unlike Mueller, who could in principle be fired, the Southern District isn't one man; it's a whole office of career lawyers. It can't be fired. Even if Robert Khuzami, the acting U.S. attorney in this case, were removed, no new U.S. attorney could realistically call off the prosecutors.