If it is possible to be more pessimistic than blueDC, I will provide that pessimism. Completing this connection in the 70 minutes required is impossible. There just isn't enough luck in the world for eveything to line up to make this connection, unless your into flight into IAD arrives early, and your flight out of DCA closes the door late.

Everything must be perfect in both airports as well as on the roads. That just isn't going to happen.