Being watched by the FBI, especially by the higher-ups in a high-level department like the Manhattan office of New York, Pratt Senator will definitely not have an easy time. It's still in the investigative stage now, but when it's time to act, it'll surely be a major operation.
Peter and his team are looking for evidence from when Senator Pratt was a dirty cop back in the day. It's in a evidence box Neil Cavrey's father and his partner stashed away over a decade ago. The evidence should be enough to strip the Senator of his protective cover. Without the title of Senator for protection, Pratt is just a nobody.
Ruiz is investigating gang elements related to Pratt, and there must be some. As a former police chief of Washington, D.C., over the years, rising to a Senator must have been supported by both the underworld and the legit side.