I've always taken this to be the -te form of だ. My reasoning is similar to the discussion given here:
However, I ran this by a native speaker, and she strongly felt that the で should be understood as an abbreviation of ・・・の中で. (But perhaps that で is itself still understandable as the -te form of だ. In fact, I have the feeling that it is.)
I'd love to hear your thoughts on this.