Matt Wieters, right, celebrates his two-run home run with Steve Pearce as Nationals catcher Wilson Ramos looks during the eighth inning Thursday. The Orioles’ 5-4 completed a three-game sweep at Nationals Park. (Nick Wass/Associated Press)

Loss by loss, chance by missed chance, the Washington Nationals whittled their season to games like this, a 5-4 loss to the Baltimore Orioles to finish a sweep on a quiet, early-autumn afternoon at sleepy Nationals Park. They are seven and a half games back in the National League East now with 10 to play, playing near-meaningless games in front of unexcited crowds, their playoff odds growing as long as the afternoon shadows that crept across the field by Thursday’s first pitch.

The Nationals fell behind early, climbed back, then fell apart again — a narrative that could double as the story of their season, just a few more losses from officially becoming a disappointment.

Thursday’s bullpen culprit was Blake Treinen (2-4), who allowed a two-run homer to Matt Wieters in the eighth to turn a one-run lead into the decisive deficit. Center fielder Michael A. Taylor chased that ball a few steps before slowing. He stood still to watch the high line drive travel into blue seats in right-center field, helpless against the inevitable. The Nationals now have blown a lead in the sixth inning or later seven times in their past 16 games.

“With the exception of what I wasn’t able to do tonight, I don’t think you can weigh an opinion on the fact that we’ve been doing bad,” Treinen said. “I think we’ve been doing a really good job of executing pitches, getting outs. Tonight didn’t go our way. I need to do a better job of focusing and making better pitches.”

The Nationals gave themselves a chance in the bottom of that inning but could not turn a runner in scoring position into the tying run. Clint Robinson doubled to lead off. Ian Desmond bunted right in front of home plate. Wieters threw Wilmer Difo, Robinson’s pinch runner, out at third.

“Desi tried,” Manager Matt Williams said of Desmond. “The ball’s running away from him. It went off the end of the bat, and he didn’t get enough of it to get it out there.”

In July in Baltimore, this neighborhood “Battle of the Beltways” grudge match was injected with energy. In late September, in the District, theatrics were subdued by the teams’ transformations from contenders to seeming pretenders.

Thursday’s series finale, a rescheduled affair forced by the postponement of Monday’s game, was played in front of a crowd announced at 28,456 that looked more sparse. Those in attendance thought they might see a brawl, might see Bryce Harper get hit after Jonathan Papelbon hit Manny Machado the previous night. Instead, they saw a losing effort, and while many teams look lifeless in defeat, the Nationals in particular offered few signs of desperation.

“Disappointing that we didn’t win these games. But we have more to play. We have one tomorrow. Had a lead and couldn’t hold it today, and that’s baseball,” Williams said. “It happens.”

The Orioles opened up a 3-0 lead against Nationals starter Tanner Roark by the fourth. Meanwhile, Baltimore rookie right-hander Tyler Wilson held the Nationals’ offense scoreless into the fifth. He held down Harper and Jayson Werth, the Nationals’ third and fourth hitters, who finished the series a combined 0 for 18, though Harper walked seven times. Harper came to bat with two outs in the ninth. He grounded out to Machado, playing near second base in the shift. Harper went three games without a hit for the first time all season.

“It was a tough series,” Roark said, “but we’re not out of it yet.”

After Wilson Ramos singled to lead off the fifth inning, Trea Turner came to bat as the song “Look Ahead” by Future played. He doubled down the left field line, his fourth career hit, his first for extra bases. When Yunel Escobar singled to left field two batters later, Turner shot home, showing off the speed the Nationals hope will spark their lineup. Ramos tied the game with a home run hit nearly over the left field seats. The Nationals announced the blast was 430 feet, one of the deepest home runs hit that way in the history of the stadium. Fans had to get up to chase the ball because the seats out there were largely vacant.

Matt Williams won NL Manager of the Year in 2014, but the Nationals fell short of playoff expectations last season and have underperformed this year. Is Williams in danger of losing his job if the team misses the postseason? (Post Sports Live/The Washington Post)

When Escobar doubled in Matt den Dekker with the go-ahead run in the seventh inning, there were cheers. When Anthony Rendon was thrown out at home plate trying to score, there were groans. When Treinen allowed the home run to Wieters, there was quiet. The Nationals have lost seven games this season in which they led after seven innings, though to those watching, it feels like more.

“It’s part of the game,” Williams said. “Guys can go into periods where they are not getting outs like they want to.”

The Nationals could have picked up ground on the Mets, who had just completed a 3-6 homestand that included losing two of three to sputtering Atlanta. Instead, the Nationals dropped three games at home, where they have won 14 series and lost 12, and the Mets rebounded to beat the Reds. The Nationals play one more series here, this weekend against the Phillies, during which they may statistically stamp out hope.

“Frustrated,” Desmond said. “But as long as we have a chance, there’s still hope.”

Music still blares, fans still react and players still celebrate — but something is gone from the air at Nationals Park now, unlikely to return until spring.