Depending on how long they considered it imminent, politicians would react by acting in certain predictable patterns:
They'd spend money like they never expect to have to repay it.
They'd buy as many bullets as they could, arming all of the agencies they have under control to the teeth.
They'd throw a lot of money to friends they think could help get them to safety.
They'd get themselves houses in places that would be least affected, like Hawaii.
They'd ignore the small stuff--foreign policy, budgets, etc, since "what difference does it make?"
And maybe they will declare war against Russia, arrest those in the USA that take sides with Russia and will place them in internment camps labeled as traitors.