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WASHINGTON — Woof.

It was all set up Thursday for the Mets to win a second straight series rubber game — an accomplishment for a team that has floundered for most of the last month — but somehow this underachieving bunch managed to flatline.

Pitching, both starting and relief, faltered.

Bats that earlier looked ready for a breakout afternoon fell silent.

Lowly or not, the last-place Nationals kept fighting, while the team allegedly competing for a postseason berth stopped.

The Mets took a three-run lead into the fifth inning then cratered, surrendering nine unanswered in a 9-3 loss at Nationals Park that sliced their lead on Cincinnati to a half-game for the NL’s third and final wildcard spot.

The Mets’ deficit on Philadelphia for first place in the NL East swelled to seven games.

“It ain’t late, but it ain’t early, either,” Juan Soto said of the 35 games remaining. “We have to win today. We don’t have to be focused on what is going to happen in the future or what happened in the past. We have to focus on what we can do today.”

So much for the small step forward the Mets took last weekend by winning two of three games against the Mariners.

They now head to Atlanta for a rematch against a team that feasted on Mets pitching in winning two of three games last week at Citi Field.

“We have got to get going,” manager Carlos Mendoza said. “We have got to see results. We don’t have much time left. We have got to play better.”

Sean Manaea was close to untouchable early, but got sloppy in the middle innings and departed in the fifth.

The left-hander allowed four earned runs on three hits and one walk with two hit batters over 4 ²/₃ innings and was removed after 91 pitches.

The Mets haven’t seen anything close to the Manaea who carried the rotation last season.

The lefty owns a 5.15 ERA in his eight appearances this season, which includes two starts in his past three in which he failed to complete five innings.

“I feel good,” Manaea said. “I really have no explanation for why I am hitting the wall.”

Francisco Lindor homered leading off the game, setting a franchise record in the process.

The blast was his eighth leadoff homer this season, which eclipsed the mark he shared with Curtis Granderson (who twice hit seven).

Starling Marte smashed a homer in the third that extended the Mets’ lead to 2-0.

It was the seventh homer this season for Marte and third in his past seven games.

The Mets loaded the bases in the fourth against MacKenzie Gore and received a sacrifice fly from Hayden Senger for the rookie catcher’s first career RBI.

Brett Baty and Cedric Mullins each singled in the inning, sandwiched around a walk to Tyrone Taylor.

But the game went downhill for the Mets from that point.

Manaea surrendered a run in the fourth on Dylan Crews’ RBI groundout.

The lefty allowed three additional runs in the fifth after Pete Alonso’s throw to second was late trying to nail lead runner Brady House.

Jacob Young reached first on the grounder, and after CJ Abrams got drilled to load the bases, Paul DeJong’s sacrifice fly sliced the Mets’ lead to 3-2.

Chaparro walked to reload the bases before Tyler Rogers entered and allowed a single to Adams that brought in the tying and go-ahead runs.

Young’s RBI single in the sixth against Rogers extended the Nationals’ lead to 5-3.

Daylen Lile singled and stole second in the inning before Young delivered with a two-out shot off Mullins’ outstretched glove while diving.

Ryne Stanek dumped kerosene on the fire by surrendering four runs in the eighth.

House stroked an RBI single before James Wood delivered the haymaker with a three-run homer.

“There’s a lot of different parts, but I think it starts with our starting pitching,” Mendoza said of the team’s slide. “They set the tone, and when they go, the whole team pretty much goes. At this point, everybody is pretty much healthy. We need to be better.”