 In the middle of the measure, before the trumpet "fanfare", the chord alters to C♯dim7 instead of remaining on C. This leads to a G7 chord (rather than to a D minor chord), and is an example of a common-tone diminished seventh chord.This chord "leads into" the V chord (G), then to a D7 chord.Here a D chord in the key of C would be the ii chord (as all ii chords must be minor).