After replacement keeper Milos Kocic conceded Levesque's score in the 86th minute, United got a goal back when Clyde Simms steered a deflected free kick past Kasey Keller in the 89th but couldn't get an equalizer despite pushing into the Sounders' half of the field repeatedly during five minutes of stoppage time.
Both teams came through the MLS play-in round (two games) as well as the regular bracket (three games) to reach the final. In those five-game runs, lineups of backups and reserves had been fielded many times, but both Seattle coach Sigi Schmid and D.C. United coach Tom Soehn played as many starters as were available.
Schmid paired Tyson Wahl and Pat Ianni in central defense with Jhon Kennedy Hurtado suspended and Tyrone Marshall injured, and for most of the game the Sounders looked the more secure team.
A noisy crowd of 17,239 - spurred in part by a very public spat between rival GMs Adrian Hanauer and Kevin Payne regarding bids submitted to host the game - kept up a constant din, with a few hundred Seattle supporters punctuating their team's good moments with cheers and chants.
Until he callously and deliberately stepped on Montero's stomach as the young Seattle forward lay on the ground, Wicks had stymied the Sounders with some help from his teammates. He kicked away point-blank shots by Montero and Ljunberg, and defender Dejan Jakovic swept a ball away from rookie Steve Zakuani as he tried to burst through the left side.
Wicks saved again in the final seconds of the first half, slapping away a Montero header moments after Keller had repelled a Ben Olsen shot swerving toward the top corner.
Soehn brought on Santino Quaranta for Fred at halftime and a spell of United pressure nearly produced a goal in the 60th minute when Christian Gomez, Chris Pontius and Luciano Emilio weaved their way into shooting range, but Emilio's weak shot belied the buildup and Keller saved easily.
In the next Seattle attack, Wicks took a seemingly innocuous knock from Montero. Six minutes later, Keller started a Seattle surge upfield that produced a Zakuani header that Ljungberg outpaced Jakovic to stab on goal. Wicks got his left glove on the low shot but it ran free for Montero to bang into the net, after which he tumbled over the keeper, and as he rolled over, Wicks stepped on and over Montero.
After a few minutes of protests and discussions amongst the officials, Prus brought out the red card, and Wicks - who'd argued fiercely with teammate Marc Burch during their league game last weekend - left the field. Kocic replaced Gomez.
Sebastien Le Toux set up the second goal as he and Jakovic dueled for a bouncing ball on the right side of the penalty area. Burch, not sensing any danger, failed to provide cover, and when Jakovkic fell LeToux walked the ball along the goal line before centering a pass that a wide-open Levesque, who had replaced Zakuani in the 74th minute, couldn't possibly miss.
Simms scored when a Quaranta free kick hit a player in the wall and deflected 90 degrees right into his path. His first-time redirection gave Keller no chance, but in the final minutes the keeper snagged a couple of crosses and his teammates' clearances did the rest.
Seattle thus emulates the feat of the Chicago Fire, which won the Open Cup - as well as MLS Cup - as an expansion team in 1998. The Sounders resume their quest to make the playoffs a week from Saturday with a rematch against D.C. United at RFK. United plays twice in the interim: at Dallas on Saturday and at home against Kansas City next Wednesday.
Lamar Hunt U.S. Open Cup, Final
Sept. 2 in Washington, D.C.
D.C. United 1 Seattle Sounders FC 2. Goals: Simms (Wallace) 89, Montero (Ljungberg) 67, Levesque (Le Toux) 86.
D.C. United -- Wicks, Namoff, Jakovic, Burch, Fred (Quaranta, 46), Simms, Olsen (Wallace, 82), Pontius, Gomez (Kocic, 71), Moreno, Luciano Emilio.
Seattle -- Keller, Riley, Ianni, Wahl, Gonzales, Ljungberg, Alonso, Vagenas, Zakuani (Levesque, 74), Montero (Evans, 78), Le Toux.
Yellow Cards: Seattle -- Ianni 24, Gonzales 49, Vagenas 79.
Red Card: D.C. United -- Wicks 69.
Referee: Alex Prus.