Frank Capra and James Stewart were nearly unsurpassed at the task of taking the kind of story that is optimistic but that borders on being trite, and making it into a satisfying, worthwhile movie. In "Mr. Smith Goes To Washington", they accomplish this with a little help from Jean Arthur, Claude Rains, and Edward Arnold. It's not quite on the level of "It's a Wonderful Life", but it is as good as almost anything else of its kind.
Stewart's performance is important right from the beginning - hardly anyone else could have been believable as the earnest unknown who suddenly becomes an important political figure. Even his wide-eyed appreciation for what he sees in Washington comes across believably. As the story gets more complicated and his character is developed further, Stewart is even better.
The secondary characters are also important, because the story itself is a rather stylized, though still worthwhile, statement about politics. The characters are more believable than are many of the plot developments. Rains contributes a lot as Stewart's troubled colleague, and Jean Arthur is a natural for this kind of role. Arnold plays his devious character well. Capra holds it all together with his craftsmanship, keeping the story on track and getting the most out of the situation.