Covington & Burling, at the top of The National Law Journal’s list of Wash­ington’s largest firms for the second consecutive year, added about 15 total lawyers last year in a time when elite firms, especially ones that focus on litigation, find the most ability to expand in Washington.

Call it the muddying of the middle: Legal business around Washington has become increasingly concentrated around the premium, largest law firms as midtiered offices get bogged down, unable to grow or assert themselves in the competitive capital city.