At that time, McCarthy had been in the Senate for all of five months, not nearly long enough for him to start grilling people for having associations with the Communist party.

Of course, few communists openly come out and say they are making communist propaganda. It's a bit silly to think Capra would have been one of them, or that "It's a Wonderful Life" doesn't also celebrate the banker George Bailey (portrayed by conservative Republican Jimmy Stewart), but I can understand why the government would want to monitor communist groups, even if they get it laughably wrong from time to time.