If you think about the fact that the movie was released 2 months after the start of World War II, you can almost predict that director Frank Capra had already begun to catch wind about the situation in D.C. by the time of filming. The film already was going to cause controversy because it was one-sided and it was bent on showing the Senate as a corrupt political body that is full of nepotists and monopolists.
This being said, you could argue that the scene where Senator Paine begins to go crazy and starts yelling out the truth was a necessary addition to an otherwise dark comedy movie that would have totally bummed out a society that was about to enter a World War in less than 4 more years. However, it is also arguable that this is Capra's stylistic device to provide the much needed happy ending that the movie deserves...and some what definitely needs: You will find in the world of movie making that the producers are aware that people do not want to pay to watch tragedy.
Additionally, this last scene brings back the hope in the system, which I am sure was a hint that Capra got from the mainstream production.
Your question, however, asks whether this scene had a foreshadowing. You could say that Senator Paine, although was benefiting greatly from his plan, had already shown signs that he wanted out of the deal. He never really does much about actually doing it, but the slow transition into madness is a perfect ending to a movie that is perfectly acted by James Stewart's physical and neurotic-type comedic style.
So, to make the long answer short, James Stewart does show a side in his character that hints at the fact that he could not possibly fall too low in the great scheme of things. He does request, at one point, to be let out of the scheme, and his comical nature makes us almost want to trust that he will change his mind.
Yet, it is the surprising element of his action what brings even further comedic value to this awesome plot, and what brings the refreshing ending to a very thick and delicate story.