Amazingly, the Nats have never lost to the Mariners - they swept a 3-match series at RFK on 10th-12th June 2005 (winners Ayala, Patterson, Armas in that order) and an away series at Safeco on 13th-14th-15th June 2008 (winners Hill, Clippard, Colome - I also distinctly remember Kory Casto hitting his first major-league home run in that series).
...oops I think I just jinxed our lads....