Andres Romero opened the scoring in the 57th minute and, after Jeff Larentowitcz saw red seven minutes later, Marco Di Vaio doubled the lead (76).
It took the impact ahead of Sporting, who slipped to third courtesy of a 3-2 home defeat against the Portland Timbers.
KC twice led through Chance Myers, who scored in the first and 29th minutes, but twice the Timbers levelled courtesy of Ryan Johnson (24) and Darlington Nagbe (33).
Rodney Wallace sealed the points for Portland in the 58th minute.
A double from Tim Cahill gave the New York Red Bulls a 2-1 win at Toronto FC which took them above Sporting and into second.
Cahill opened the scoring in the 39th minute and, after Jonathan Osorio stepped off the bench to equalise (83), sealed the points with an 89th-minute winner.
Western Conference leaders FC Dallas were held to a 2-2 draw at the Vancouver Whitecaps.
Dallas appeared to be cruising to victory when an own goal from Andy O'Brien in the ninth minute and Matt Hedges' effort two minutes into the second period put them two goals up.
But substitute Kekuta Manneh pulled one back in the 72nd minute and Camilo Sanvezzo equalised three minutes later, before Dallas midfielder Jackson was sent off in the 78th minute to add insult to injury.
Goals from Mike Magee in the sixth minute and Charlie Rugg (13) gave the Los Angeles Galaxy a 2-0 win at Real Salt Lake, leaving them six points behind Dallas with two games in hand.
First-half goals from Domenic Oduro (15), Josh Williams (26) and Federico Higuain (45 pen) gave the Columbus Crew a 3-0 win over DC United.
Diego Fagundez (61) and Lee Nguyen (72) scored in the New England Revolution's 2-0 win over the Philadelphia Union, while Chivas USA and the San Jose Earthquakes drew 2-2.