Two goals in quick succession in the first half helped propel the Chicago Fire back into a coveted playoff berth.
Chicago struck twice in three minutes to tale a first-half lead. Jeff Larentowicz scored the first, beating Joe Willis from a tight angle in the 24th minute to finish off a run by Juan Luis Anongono which carved open the United back line.
The Fire then doubled their lead in the 26th. Mike Magee's long free kick hit a body in front of keeped Joe Willis and fell into a dangerous area, and central defender Bakary Soumare was there, ready to blast home a low shot.
Quincy Amarikwa ended any hopes of a miracle comeback with a goal in the 89th minute, heading home a Dilly Duka cross following a short corner..
The win puts the Windy City side (12-12-7) back into the fifth and final playoff spot, but only just - on a one-point lead over Philadelphia, which has a game in hand.
D.C. United (3-22-6) remains in last place in Major League Soccer and the Eastern Conference.