Dike scored on a second chance after Steven Smith's shot was deflected, helping the Timbers (7-15-9), eighth in the Western Conference, avoid elimination from the postseason race.
''I just tried to hit it clear and on target,'' Dike said.
Chris Pontius scored on a penalty kick in the 60th minute to give United (15-10-6) the lead. D.C. snapped a three-game winning streak and dropped to fourth place in the Eastern Conference. United began the day tied with New York for third, but the Red Bulls beat Toronto to pull into a tie with Chicago for second.
Pontius's 12th goal of the season came after a hand ball call on Timbers defender David Horst, which had several Timbers in the referee's face. Horst was given a yellow card and Timbers fans shouted boos and expletives on the officials. The officials were also booed as they left the field after the game.
Timbers owner Merritt Paulson took off after the officials, appearing to yell his disapproval and a bottle thrown from the stands came close to hitting the crew.
Horst claimed after the game that the ball went under his arm and never touched him.
''I think they were pissed off and annoyed,'' Portland general manager and interim coach Gavin Wilkinson said of his players. ''But they didn't sit back and feel sorry for themselves.''
Portland outplayed United for long stretches, had 14 shots on goal and made goalkeeper Bill Hamid work hard.
United had some close calls near the end of the first half. Dike had a clear shot that went wide after defender Chris Korb accidently passed the ball to him in the 40th minute. In the 44th, Hamid had a ball bounce out of his hands and nearly cross the goal line before he snagged it. Hamid had four saves in the first half.
The Timbers, winless on the road this season, are a tough to beat at Jeld-Wen Field, where they have won against top clubs such as Chicago, Sporting Kansas City and San Jose this season.
The Timbers play rival Seattle next Sunday. Horst said eight days was not too long to allow the frustration from Saturday's match act as inspiration for the upcoming game.
''Absolutely, next week is going to be a big battle,'' he said.
Portland had had seen encouraging play in its last three matches - a home draw with Seattle, a road draw at San Jose and a 2-1 loss at Real Salt Lake last Saturday.
Portland goalkeeper Joe Bendik started for the third consecutive game, as starter Donovan Ricketts remains out with a shoulder injury sustained against Seattle.
United was playing on artificial turf for the first time since April at New England.
- Portland Timbers
- David Horst
- Bright Dike