Chris Rolfe scored twice and Joel Lindpere had a goal and two assists before intermission as United fell to 0-7-3 away and 2-14-4 overall.
In his regular season debut with United, Luis Silva scored on a 30-yard strike in the 58th minute.
United played without its two most accomplished attackers: Chris Pontius was scratched moments before kickoff with hamstring tightness and Dwayne De Rosario missed his third consecutive league game with a hip flexor strain.
The teams will meet again at Toyota Park on Aug. 7 in the U.S. Open Cup semifinals.
United had made defensive strides in the past month, allowing five goals in six league matches. But in the second minute Saturday, Lindpere went unchallenged along the sideline before dropping a perfect cross over Daniel Woolard and onto the foot of Rolfe, who snapped a 15-yard volley past advancing goalkeeper Joe Willis. [video]
Nine minutes later, Jeff Larentowicz surged past Woolard, cut the ball back to penetrate the penalty area and set up Lindpere for a sliding finish from 14 yards. [video]
United has yielded 16 goals in the first 30 minutes of matches (17 over the final 60 minutes).
After creating mild threats, United allowed another goal in the dying seconds of the first half. Lindpere lifted a pass over the backline to Rolfe, who dashed away from Ethan White and chipped the ball past Willis. [video]
Coach Ben Olsen pulled White at halftime and inserted Dejan Jakovic, who had missed the previous eight league matches with an adductor injury.
Mike Magee converted a penalty kick in the 56th minute, his 12th of the season and sixth since joining the Fire from Los Angeles in May.
Silva’s sensational drive to the top right corner two minutes later [video] ended United’s streak without a goal in the run of play at 803 minutes. (D.C.’s only regular season goals since May 19 had come on penalty kicks.)
Meantime, 23 miles west of Toyota Park, the Washington Spirit conceded an own goal in the 63rd minute and lost to the Chicago Red Stars, 1-0, in a National Women’s Soccer League match at Benedictine University in Lisle, Ill.
The Spirit (1-11-4) is winless in 10 straight (0-9-1) and has failed to score in eight of those games. During the rut, the club has converted two penalty kicks (no goals in the run of play since mid-May, a span of 906 minutes).
Chicago (6-6-4) has defeated Washington twice in a 11-day span.
Defender Ali Krieger (concussion) did not play for Washington, which will host the Boston Breakers next Saturday night at Maryland SoccerPlex.