With one of the best wins in the franchise's short history, the Union solidified their hold on a playoff spot on Saturday night in Seattle.
Playing on the road against the league's second-best team, the Union came away with a 2-0 victory against the Sounders at CenturyLink Field.
The victory moved the Union (11-7-14) into first place in the Eastern Conference with 47 points.
Sporting Kansas City (45 points), Columbus (44), New York (43), and Houston (43) follow close behind. Each team has two games remaining in the regular season.
The Union are primed to grab one of the 10 playoff spots in MLS in just their second season of existence. The clinching can come as early as Saturday, when the Union host Toronto FC at 4 p.m. A win would get them in. They could also clinch if there is a winner in Friday's Houston-Portland game and the New York Red Bulls fall to Sporting K.C. on Saturday.