Ralph opened the scoring in the 15th minute, then added goals in 55th and 69th minute. He has six goals on the season, tying Carlos Ruiz for the league lead.
Henry Ring made five saves to earn his league-leading sixth shutout for the Fire (4-2-4).
With teenage-sensation Freddy Adu making his first appearance at Soldier Field, the contest drew a crowd of 25,810.
D.C. United (3-5-3) has lost three of its last four games.