This will be a tough conversation ! The contributions of your colleague’s department are just not good enough and it needs to be addressed. You have collected all the facts and figures. But somehow you hesitate, because you expect your colleague to become very defensive. Defuse tension and start the discussion from your objective. Why is this topic important, why is it difficult. Make clear that you want to hear your colleague’s view after you have presented yours. Some tension will remain. However, it is framed by positive intend not accusation. Big difference !
You’re under pressure because you definitely started this important project too late. How