Senator Jon Tester (D-Mont) was talking about McConnell’s demand that the Democrats promise to keep the filibuster intact. Tester noted that the Democrats are now in the majority and should not accede to McConnell’s demand. Here is the quote:
“Chuck Schumer is the majority leader and he should be treated like majority leader. We can get sh** done around here and we ought to be focused on getting stuff done,” said Sen. Jon Tester (D-Mont.). “If we don’t, the inmates are going to be running this ship.”
This is a mashup of “the inmates are running the asylum” (the people least capable of running an organization are now in charge) and I believe “rats leaving/deserting a sinking ship” (the least loyal people will be the first to abandon a project). My guess is that Tester thought of Republicans and then associated them with rats, activating this nice malaphor.