The teams remained even Sunday, as the Sounders rallied from two goals down for a 2-2 draw before a crowd of 27,244 at StubHub Center.
But the tie will be broken and superiority settled starting at noon Saturday when the Sounders and Galaxy meet once again, this time in the final game of their regular-seasons and this time at CenturyLink Field, which will be open to its full capacity.
A win or a draw will give the Sounders their first Supporters Shield. A win by the Galaxy would return it to Los Angeles.
Never miss a local story.
For long minutes on Sunday it appeared that the bar would be set higher for Seattle, as the Galaxy scored late in the first half and early in the second to go two goals up.
However, Clint Dempsey started the Sounders on their way back with his 15th goal of the season, and then Lamar Neagle of Federal Way got the equalizer in the 72nd minute.
Final: LA 2, Sounders 2. Supporters Shield will be determined Saturday at CenturyLink. Win or draw brings Shield to Seattle.
88' Second yellow on Omar Gonzalez. Sounders have man advantage for final couple of minutes and stoppage time. And he's out for Saturday.
87' Barrett for Neagle.
72' GOAL Sounders. Lamar Neagle's ninth. Even, 2-2.
69' GOAL Seattle. Dempsey's 15th , after cross from Yedlin and touch from Martins. LA, 2-1.
66' Pappa for Evans. a yellow would have scratched Evans for next week through accumulation.
50' GOAL LA. Sarvas from the top of the area, again after Sounders' failure to clear after corner. Second half: ... has begun. No changes.
Halftime: LA, 1-0.
ST: ST: GOAL LA. Brad Evans heads a poor clearance directly to Hasidic at the top of the area, and he makes Sounders pay, rolling a shot just beyond the reach of Frei and into the far side netting.
36' Yellow on Dempsey for intentionally pulling down Dempsey on a break.
30' Scoreless. Run of play evening out.
26'-27 Yellow cards to Zach Scott and Gonzalo Pineda.
20' Martins jukes his way to an open angle and sends hard shot on target, but nicely saved.
First 20 minutes: Two best chances both to LA. Also seemed to be a bit of an on-pitch argument between Dempsey and Oba.
First 10 minutes: More possession on the LA attacking end, and one great chance: Donovan header hits the crossbar.
First kick: ... has been struck. We're under way. Galaxy in white/white; Sounders in pitch black.
Lineups: For Seattle: Frei; Yedlin, Marshall, Scott, Gonzalez; Evans, Alonso. Pineda, Neagle; Dempsey, Martins. Bench: bench: Hahnemann, Traore, Anibaba, Azira, Pappa, Cooper, Barrett.
For LA: Penedo; Gargan, Gonzalez, DLG. Rogers; Husidic, Sarvas, Juninho, Sonovan; Keane, Zardes.
Pregame: The Sounders and LA Galaxy meet at 5:30 PT this afternoon with plenty on the line. Most of all, Sounders FC claims its first Supporters Shield with a win, while the Galaxy will push the settling of the Shield until Saturday at CenturyLink with either a win or draw. The Sounders also could set a modern MLS record with their 20th win, while the Galaxy are going for a club record 13th home win.
Coverage begins at 5 p.m. on ESPN2. We'll also keep the blog going before, during and after the match, with additional updates and comments on Twitter @donruiztnt. The game also will be heard on the usual radio outlets.
Here's our preview from today's TNT.