For the second consecutive home game, D.C. United gave up a tying goal in the final six minutes. This time, it was a 2-2 tie against Philadelphia. (Ned Dishman/GETTY IMAGES)

D.C. United was enjoying a merry evening — Dwayne De Rosario’s debut, Bill Hamid’s exquisite saves, Andy Najar’s ridiculous go-ahead goal — when another late-game breakdown at home cost the defensively fragile club more valuable points in the MLS standings.

A week after relinquishing a lead in the closing moments, United surrendered Carlos Ruiz’s goal in the 84th minute Saturday and settled for a 2-2 draw with the Philadelphia Union in front of 13,365 at RFK Stadium.

“Again, it’s there for us to take and we’re not taking it,” captain Josh Wolff said. “We’re going to look back in September or October, and these are points you can’t make up for. It’s a game we have no business giving away.”

United (4-5-7) has tied four of the past five matches, but more troubling is the inability to win in Washington and the lack of defensive resolve in the late stages. An 89th-minute equalizer marred last week’s 2-2 draw with the visiting Houston Dynamo.

“There’s no excuse to lose a game in the last 10 minutes,” rookie defender Ethan White said. “We tied, but it feels like a loss.”

With a grand opportunity to pull within five points of the first-place Union (7-4-6), United remained stuck in the middle of the muddled Eastern Conference and fell to 2-2-5 at home. Since the season-opening victory over the Columbus Crew, D.C. has won just once at RFK.

Coach Ben Olsen tried to accentuate the positive, saluting Najar’s tremendous strike and De Rosario’s integration into the lineup after two training sessions since being acquired from the New York Red Bulls.

“If that’s the number one team in the East, we’ve got a chance, we’ve got a good chance,” he said. “We’ve just got to find ways to close out games at home.”

With De Rosario in the middle, United finally had a true fulcrum. He set up United’s first goal and seemed comfortable orchestrating the attack.

In the 44th minute, De Rosario made a timely run, collected Najar’s pass on the left side of the box and, with Brian Carroll pursuing, surged toward the end line.

Union defender Carlos Valdes provided cover, leaving Josh Wolff unmarked. De Rosario released the ball before Valdes arrived and Wolff stabbed it past Faryd Mondragon from five yards for his fourth league goal.

Philadelphia’s sustained pressure after halftime got to United’s young back line. Attempting to clear Veljko Paunovic’s service in the heart of the box, first-round draft choice Perry Kitchen clumsily volleyed it into his net.

Chris Pontius nearly restored the lead two minutes later, but Mondragon made just enough contact to send it off the far post.

In the 58th minute, Najar, 18, scored his wonder goal from more than 30 yards. He collected the ball at midfield, cleverly touching it away from Danny Mwanga and dashing into space before smashing a drive with purpose and pace that tailed away from the fully extended keeper and crashed into the far side of the net.

“I can’t really believe it,” he said through an interpreter.

United remained loose in the back, however, and Hamid was called upon to make his second terrific save of the night, a reflex stop on Ruiz’s close-in header.

The breakdown came in the 84th minute. Sebastien Le Toux set up right back Sheanon Williams for an overlapping run past the late-reacting Pontius. “I was awful tonight,” Pontius said.

Ruiz snuck behind White and redirected Williams’s cross past Hamid for his fifth goal of the year and 87th of his MLS career.

“They weren’t really breaking us down,” De Rosario said. “We basically broke down ourselves.”

United notes: Center back Brandon McDonald, acquired Monday from San Jose, played all 90 minutes. . . .

Rookie Chris Korb was initially included in the starting lineup in place of left back Daniel Woolard (back spasms), but after loosening up during pregame, Woolard was cleared to play. . . .

The clubs will play a reserve league match at 10 a.m. Sunday at the RFK training grounds. Admission is free. Jamaican defender Adrian Reid, on a week-long tryout, is expected to start.

FC DALLAS 2, COLUMBUS 0: Brek Shea and Jackson Goncalves scored second-half goals to lift host FC Dallas over the Crew.

FC Dallas won its third straight game and goalkeeper Kevin Hartman finished with five saves.

It was his second straight shutout and ninth this year.

In the 58th minute, Shea scored after he rebounded a missed shot by Marvin Chavez and tapped a shot into the center of the net from the top of the goalie’s box.