United defender Emiliano Dudar, far left, watches the final minutes of the team’s season unfold. United needed multiple goals to defeat Houston; instead, it had to settle for a 1-1 tie that sends Houston to MLS Cup for the fourth time in seven years. (Toni L. Sandys/The Washington Post)

D.C. United was already facing a monumental task entering the final leg of MLS’s Eastern Conference finals Sunday, and yet before the first brigade of banner-carrying supporters turned RFK Stadium into a festival of sights and sounds, the challenge grew taller.

The leading scorer wasn’t going to play. The top defender couldn’t start. A midfielder was a late scratch, altering the starting lineup. The goalkeeper was questionable until an hour before kickoff.

United played with the same spirit that had carried the limping club into the playoffs for the first time in five years and within an improbable step of MLS Cup. But, as team President Kevin Payne conceded in a solemn locker room, “There is only so much you can overcome, so many blows.”

Needing to win by multiple goals, second-seeded United settled for a 1-1 draw with the fifth-seeded Houston Dynamo and lost the two-game, total-goals series on a 4-2 aggregate in front of 20,015 raucous spectators.

Houston will play the Los Angeles Galaxy for the championship on Dec. 1. Los Angeles advanced, 4-2 on aggregate, following a 2-1 loss to host Seattle on Sunday night in the Western Conference finals.

The biggest blow was Boniek Garcia’s goal in the 33rd minute, which extended the Dynamo’s overall advantage to three in the series. Branko Boskovic equalized in the 83rd minute — too little, too late for a United squad that had dropped last weekend’s opener in Houston, 3-1, and could ill afford to fall behind at home.

“At some point, it’s just not going to be good enough down the stretch if you have injuries and you’re not playing with your full squad,” Coach Ben Olsen said. “But I am glad we hung on this long. It shows the spirit this group has because things have not been easy. There have been obstacles after obstacle, and these kids kept pushing through.”

Top scorer Chris Pontius, who injured his groin 12 minutes into the series opener, wasn’t in uniform.

“He was done,” Olsen said. “He would have been a question mark for the finals” in two weeks.

Center back Brandon McDonald was shelved by a calf injury. Right back Andy Najar was serving a league-imposed suspension. Midfielder Lewis Neal was slated to start but was scratched with a sore thigh, reducing him to a second-half role. Goalkeeper Bill Hamid almost didn’t play because of a back ailment.

Dwayne De Rosario, United’s most important player, did return from a two-month layoff caused by a knee injury, entering in the 62nd minute to a rousing ovation. Although the 2011 league MVP invigorated a desperate attack, United squandered several opportunities and, for the ninth time in the last 10 matches, failed to score more than one goal.

“The whole second half, we were in the their end,” defender Robbie Russell said of his team hording 69 percent of possession after intermission. “There were chances to win it.”

Hamdi Salihi, the second of three substitutions early in the second half, bungled two clean runs on goal without getting off a shot. Russell missed a short volley and Maicon Santos sent a header wide.

Boskovic leveled the score after playing a give-and-go with Santos and beating four defenders to stab a near-side shot past Tally Hall.

But by then, Houston was on its way to MLS Cup for the fourth time in seven years.

United established an early rhythm, but without De Rosario, Pontius and Najar, lacked a skillful individual to unlock Houston’s defensive scheme. The Dynamo was unfazed by the atmosphere and emotion, took United’s forays in stride, minimized mistakes and calmly probed for opportunities.

In the 34th minute, Houston’s patience paid off. Brad Davis took the initiative at the top of the box, surging into the right side and skipping over Dejan Jakovic’s slide en route to the end line.

Davis, MLS’s most reliable left-footed player, used a clever touch with the outside of his best leg, guiding the ball to Garcia for an angled six-yarder over Hamid and into the top near corner. Emiliano Dudar, McDonald’s replacement, missed his mark as Garcia met Davis’s service.

“In a game like that, where you need goals, you are going to have to not make any mistakes on the defensive end,” Olsen said. “We fouled one up.”

United was better in the second half, committing more players forward and surviving Houston’s counterattacks. Hamid made a terrific save on Mac Kandji in the 78th minute, but United’s final touches were mostly off the mark.

To their credit, despite fading hopes, United’s players continued to attack with vigor, if not style, until referee Baldomero Toledo’s whistle ended their season.

“The character the guys showed, the dedication, the unity, was fantastic,” De Rosario said, holding back tears. “It’s something to build on in the future.”

Galaxy advances

Eddie Johnson and Zach Scott scored to give Seattle a 2-0 lead but Robbie Keane was awarded and converted a penalty kick in the 68th minute that clinched the Los Angeles Galaxy’s 4-2 aggregate victory over the Sounders in the Western Conference finals and a spot in the MLS Cup final for the second straight year.

Host Seattle staged a rally trying to come back from a 3-0 deficit after getting overwhelmed in the first leg of the Western Conference finals last week.

Los Angeles will host the final.