Galaxy defender Ashley Cole tried to clear a cross from the Sounders' Joevin Jones, but Cole deflected the ball back into his own goal in the 35th minute. Jordan Morris made it 3-0 in the 44th, with Dempsey assisting.
Romain Alessandrini, who had scored a goal in three straight games for the Galaxy, missed four shots. His left-footed shot in the seventh minute was blocked, and Sounders goalkeeper Stefan Frei stopped his shot in the center of the goal in the 65th minute. Alessandrini also received a yellow card for unsporting behavior in the 41st.
The expansion United (2-4-2), who rallied to tie Houston 2-2 on April 15, has won two straight at home.
Both teams missed early scoring opportunities in the first half. Ibson flicked a header from the edge of the 18-yard box, but a charging Christian Ramirez didn't get a foot on the ball and the shot missed to the right in the tenth minute. Kevin Doyle's left-footed shot from outside the box for Colorado sailed over the crossbar in the 17th.
Goalkeeper Zac MacMath made three saves for the Rapids (1-4-1). MacMath started in place of Tim Howard, who is serving a three-game suspension for profane language toward a fan during a game against Sporting Kansas City on April 9 and an altercation with a fan following the match, which Colorado lost 3-1.