Others have said it all, and I can only second their enthusiasm.
Tim is informative, attentive to his group, a master at presentation and managing a moving group, all the while maintaining a relaxed manner. He is genuinely interested in sharing and relating to his group. Over 50 years of traveling, I have not seen anyone more skilled at leading and informing a tour group.
If all you do in Washington is take this tour, it will have been a worthwhile visit.