One night after walking off in dramatic style with a victory over the Phillies, the Nationals were shut down by Philadelphia’s Roy Oswalt, who tossed eight shutout innings, striking out nine. (Greg Fiume/Getty Images)

The red T-shirt rested on a chair inside the Philadelphia Phillies clubhouse, technically the visitor’s locker room at Nationals Park. The shirt displayed the date and the teams, and underneath bold letters spelled out, “Citizens Bank Park South.”

If and when the Washington Nationals are jousting with the Phillies at the top of the National League East, they may look back on Saturday night as one the low points in their imbalanced rivalry. The Nationals had won four of five meetings, before the Phillies restored order with a 5-0 victory as their fans’ latest invasion of Nationals Park led to Washington’s first sellout this season and set a stadium attendance record of 44,685.

The crowd surpassed the old mark, set June 25, 2009, against the Boston Red Sox and their rabid followers, by 2,700. Phillies fans chanted players’ names, roared with each Philadelphia run and booed Jayson Werth for having the audacity to catch a flyball in right field or walk into the batter’s box.

When the game ended, the seats remained about 85 percent full, nearly all of them occupied by Phillies fans. Thousands had come by the busload, and now they stood and clapped as the final out beckoned. Nationals players could not help but notice.

“Of course,” first baseman Michael Morse said. “They’re cheering for the other team. You wish they were cheering for you.”

Said Manager Davey Johnson: “I mean, I love to see a packed ballpark. Hopefully, they’ll be rooting for us and there won’t be any room for those other guys. The Phillies have good fans. They love their baseball. And we didn’t do much to get our fans cheering.”

Phillies starter Roy Oswalt dominated for eight innings, striking out nine and handing the Nationals their 12th shutout loss this season. John Lannan pitched effectively for five innings, but came undone before a quick hook in the sixth and absorbed his 12th career loss against the Phillies.

A week before in Philadelphia, the Nationals made a mess, allowing seven unearned runs in an 11-3 loss. In Saturday’s rematch between Lannan and Oswalt, the Nationals did not reach those depths. But several crucial mistakes, along with their punchless offense, created another defeat.

Ryan Zimmerman provided a bright spot, going 3 for 4 the night after his walk-off grand slam stunned the Phillies. He raised his batting average to .364 since the all-star break and made a handful of slick plays at third base, the latest evidence he has fully recovered from surgery to repair a torn abdominal muscle.

The Phillies scored first with a two-out rally in the fourth, aided by a defensive blunder by shortstop Ian Desmond. Lannan rolled past Ryan Howard and Hunter Pence before John Mayberry roped a single to left. With Carlos Ruiz at the plate, Lannan picked off Mayberry at first.

Desmond had stationed himself far from second base, expecting Ruiz, a right-handed hitter, to pull the ball. In hindsight, Desmond said, he should have let second baseman Danny Espinosa cover second. Instead, he sprinted to the base and had to catch Morse’s perfect throw at full speed. And he dropped it.

“I was trying to pick up the ball with my head moving,” Desmond said. “It was just one of those things.”

Said Johnson: “He just took his eye off it. You can’t do that. This ballclub or any good ballclub, you can’t do that when you have a chance to get out of the inning.”

With the inning still alive and Mayberry on second, Lannan walked Ruiz, his third walk of the game. Wilson Valdez drilled a line drive into the right-field corner. Before Werth could dig the ball out, both runners scored and Valdez slid into third with a triple.

“I had to get the ball down,” Lannan said. “And I didn’t in key situations.”

The Nationals had a chance to respond in the bottom half of the inning. With men on first and second, Werth swatted a single to left field. As Mayberry charged, third base coach Bo Porter windmilled Zimmerman home. Mayberry fired a one-hop dart to Ruiz, who tagged Zimmerman out just as he slid in.

“I like being aggressive,” Johnson said. “I’ll never get on him for being overly aggressive. The only thing on that play I was upset about was that the trail runners didn’t take advantage of that play at the plate by going to third and second. Morse should be more heads-up.”

The Phillies had not scored an earned run off Lannan until the sixth began, at which point they turned the game into a laugher. Pence greeted Lannan by crushing a 1-0 pitch — “a weak little slider,” Lannan said — into the left-field seats. After singles by Mayberry and Ruiz, Johnson trudged to the mound and took the ball after Lannan had thrown 82 pitches.

“Sometimes when he gets a little tired, he humps up and then the ball comes up,” Johnson said. “He tries to get a little extra on it, and that brings it up.”

Reliever Collin Balester could have escaped with minimal damage, but when Oswalt dropped a one-out sacrifice bunt, Morse scooped the ball and fired to third base, an ill-advised decision that loaded the bases. Jimmy Rollins, the following batter, singled in two runs.

Saturday night offered a reminder of how far they have to go, but the Nationals have made progress against the reigning kings of their division. They can still win this series, even. All they have to do Sunday is beat Roy Halladay, maybe in front of the other’s team’s fans.

“Their fans are unbelievable,” Lannan said. “They come wherever they play. We had a record crowd tonight. Hopefully, someday the crowd’s that big and they’re all cheering for us.”