A goal from Danny Califf gave Chivas USA all it would need to snatch three points on the road.
Ben Zemanski bent a free kick into the Portland box where Califf was strong in the air and headed home, breaking open the deadlock.
Darlington Nagbe, David Horst and Kalif Alhassan all had decent chances to pull the home side back into the match, but none of them could find the net.
The loss leaves Portland at the bottom of the Western Conference while the win moves Chivas USA within striking distance of a playoff spot.
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