At Delta's website, you can select the option for "SkyMiles Award Ticket" under "Book a Trip." Award trips on both Delta and Northwest should appear on the display. However, Delta miles are also redeemable for travel on Alaska Airlines, which is a Delta partner but not a SkyTeam partner, and Alaska offers nonstop service between SEA and DCA. I believe that you have to call Delta to book an award trip on Alaska. Ask Delta to check award availability on Alaska for the dates in which you are interested. You may even find that you can fly nonstop in one or both directions for a 25,000-mile roundtrip redemption.