The weather slammed down on PPL, but the fans who came out were rewarded with a chaotic game and a dramatic ending as Philly moved into second in the East.
Chivas USA scored first, in the 14th minute. Union defender Amobi Okugo slipped on the soaked grass, and Zac MacMath rushed out to cover, allowing Jorge Villafana to slide a simple pass to Jose Correa, who had an easy finish to make it 1-0.
The Union equalized in the 58th minute, Brian Carroll getting the last touch on Sebastien Le Toux's cross to level the scores at 1-1.
The Union lost manager John Hackworth in the 77th minute, when he was dismissed by referee Jorge Gonzalez.
Hackworth was soon followed by Chivas USA's Josue Soto, who was shown a second yellow for dissent after a pass back to the goalkeeper was called against Chivas USA just seven yards from goal. Sebastien Le Toux laid the set piece off to Michael Farfan, who blasted his shot into the roof of the net and gave Le Toux the league lead in assists.
Conor Casey ensured all three points late on, tapping home a cross - from debutant sub Fabinho - unmarked at the back post.
The result extends Chivas' winless streak to 14 games, and ends a three-game winless skid for Philly. The Union moves up to second place in the East.