WASHINGTON — The Mets absorbed a double whammy Monday night. Not only did Ryan Raburn’s single with two outs in the ninth inning give the Nationals a 3-2 walk-off win, but Yoenis Cespedes came up walking gingerly after sliding in hopes of making a game-saving catch.

Raburn’s sinking liner to left provided a fitting conclusion to a game that featured wild swings, thanks to two of the National League’s leakiest bullpens.

In the eighth, Michael Taylor hit a two-run homer off Jerry Blevins after what had been a scoreless pitchers’ duel through seven innings between Steven Matz and Stephen Strasburg.

Curtis Granderson came off the bench to answer in the ninth, hitting a tying two-out, two-run homer off Matt Albers. But Raburn delivered the deciding blow against Fernando Salas, the Mets’ third pitcher in the inning.

“That’s a great team over there that’s able to find ways to continue to keep putting pressure on and come out on top,” said Granderson, who hadn’t played since Saturday because of discomfort in his right hip.

Once Raburn’s drive found the turf, Cespedes popped up and immediately sent up red flags, raising concerns that he had aggravated the left hamstring injury that kept him on the disabled list. He later insisted that he was just experiencing cramps in his right hamstring, perhaps a side effect of the hot and muggy conditions.

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“It’s just a cramp,” Cespedes said through a translator. “It could happen to anybody. I could have been walking, I could have been sleeping.”

Terry Collins, however, said he is unlikely to play today.

After Matz and Strasburg duked it out, it became a battle of the bullpens, and the Nationals prevailed after the Mets let their chances slip away. The worst of it came in the eighth.

With lefty Matt Grace on the mound, Brandon Nimmo legged out an infield single and moved to second on a sacrifice bunt. Jose Reyes followed by ripping a single to left. Third-base coach Glenn Sherlock waved Nimmo around, even though he reached third base as leftfielder Brian Goodwin was collecting the ball. The throw easily beat him home.

The Mets challenged the play, contesting that catcher Matt Wie ters had illegally blocked the plate, to no avail.

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Still, they had a chance.

After a walk to Asdrubal Ca bre ra put runners on first and second with two outs in the eighth, Cespedes stepped to the plate, hoping that his slump might come to a timely end. Instead, he bounced into a forceout. He has five hits in his last 34 at-bats, none for extra bases.

Granderson had not played since Saturday with discomfort in his right hip. He was not sure precisely what had caused the symptoms, though he knew it was trouble.

On Sunday, he tried to get loose in hopes of coming off the bench as a pinch hitter, but the pain and stiffness did not allow it. Collins held out Grand erson from the starting lineup for a second straight game yesterday and expressed doubts that he would play today. Then Granderson gave the Mets new life with one swing, only to watch it vanish.

The Nationals’ bullpen entered play with a 5.12 ERA, the worst in the National League. But the Mets were second-to-last with a 4.87 ERA, and that fact came back to haunt them. Blevins walked Raburn before Taylor’s homer, and Paul Sewald and Josh Edgin each issued a walk before Raburn’s winning hit.

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The Mets fell to 38-44, 10 1⁄2 games behind the Nationals and 9 1⁄2 games out of a wild-card spot with five games left before the All-Star break.

Said Collins: “Right now, we’re not getting it done.”