Michael Morse douses Bryce Harper after the Nats finish off the Mets. (Patrick McDermott/Getty Images)

Afterward, in the Washington Nationals’ clubhouse, Bryce Harper sniffed at the shirt he had worn under his uniform. It is still the beginning for Harper, still the time in his career when every night may bring another first, when every detail matters for posterity. He wanted to discern which liquid he had been doused with after the first walk-off hit of his career. “Yeah,” he said, smiling. “It was Gatorade.”

Harper had come to the plate in the 12th inning Tuesday night at Nationals Park, with two outs, the score tied and the bases loaded, a chance to end an impossibly wild game after more than four hours. Down to his last strike, Harper drilled a sinking line drive to left field that gave the Nationals a 7-6 victory over the New York Mets, completing his team’s third comeback of the night and making him the first teenager since 1988 to smack a walk-off hit.

The Nationals needed to so much to happen before they could mob the 19-year-old who won’t act his age. They had blown a three-run lead. Ian Desmond drove in three game-trying runs. The Nationals fell behind in the 10th inning on a wild pitch from a reliever. They fell behind again in the 12th when Ross Detwiler, the eighth pitcher they used and the last one out of their bullpen, gave up a home run. But they would wake up in first place Wednesday morning because of Harper.

“I don’t think of him as a 19-year-old kid, but that’s exactly what he is,” Detwiler said. “He’s going to be around a long time. It’s pretty cool to see the beginning.”

The rally that led to Harper’s hit started against Elvin Ramirez, a Rule 5 pick the Nationals returned to the Mets this winter. Michael Morse led off the 12th with a double off the out-of-town scoreboard. Desmond delivered his biggest hit, chopping a double down the third base line, and Morse raced home with ease.

After Danny Espinosa flied out, Desmond moved to third on Ramirez’s wild pitch. With Detwiler on deck and no more relievers in the bullpen, Ramirez intentionally walked Jesus Flores — and in the process, in an ode to the nature of the game, nearly threw ball three over catcher Josh Thole’s head.

The move backfired when Ramirez could not throw three strikes to Detwiler, perhaps the Nationals’ most hopeless hitter. Detwiler’s walk loaded the bases for Xavier Nady. He worked the count to 3-2 before hitting roller to first baseman Ike Davis, who cut down the potential winning run at the plate.

The game had come to Harper. In the 11th, Ramirez had struck out Harper chasing a pitch out of the zone. Harper still treated his first two strikes as if they were found money, an opportunity to see a pitch he could maul. He took a vicious hack and swung through Ramirez’s first pitch. He took another fastball for strike two.

Harper spread his stance a little wider and told himself to just use his hands. He took the same approach with two strikes in the third inning, when he flared a single to left to drive in the Nationals’ first run off Mets starter Chris Young. Harper knew Ramirez could not walk him, and so he expected a strike, even down 0-2.

“He doesn’t get caught up in the moment,” Nationals third baseman Ryan Zimmerman said. “For how young he is, he does a really good job at that.”

Ramirez rifled a 96-mph fastball, outside and at the knees. “I got a pitch I could handle a little bit and just threw my hands at it,” Harper said. The ball rocketed into left field, sinking as Vinny Rottino charged. He dove. Harper sprinted to first and stared and hoped.

“I wasn’t sure if he caught it,” Harper said. “If he would have caught it, I would have freaked out.”

The ball skipped before it settled in Rottino’s glove. Harper raised his fist into the sky. Zimmerman was the first National to reach him. The face of the franchise leaped into Harper’s arms.

“Oh, my gosh, that was unbelievable,” Harper said. “To have that moment with Zim, it was pretty unbelievable. He’s been there for me through the whole thing, every time I need him.”

Said Zimmerman: “I was just happy the game was over and we won.”

The rest of the Nationals arrived soon after Zimmerman. They pounded Harper on the head. One of them splashed him with a cup of water. Harper escaped the pile, smiling and looking over his shoulder.

Harper ambled toward the Nationals dugout to conduct an on-field television interview. “That was incredible,” Harper said, still breathless. Morse dumped a Gatorade tub over his head.

In his first six weeks in the majors, Harper has defied every convention and surpassed any expectation. Nothing he does surprises his teammates any longer. His OPS sits at .903, the best on the team.

“Honestly, he’s one of the best players I’ve ever seen,” Desmond said.

“He’s a man-child,” Morse said. “This guy is unbelievable.”

Held together since spring by starting pitching, 19-year-old enthusiasm and bailing wire, the Nationals (31-22) still have not found the point at which they will crack. If it did not arrive Tuesday night, it may not exist. The Nationals took a three-run lead into the sixth inning, when starter Jordan Zimmermann’s dominant start unraveled with two homers.

The Nationals lost the lead when the Mets (31-25) rallied for two runs in the eighth against Sean Burnett and Craig Stammen, two of their most reliable relievers. Each time the Nationals needed a run to survive, they got it.

“It would have been very easy for the hitters to just say, ‘We’ll go get them tomorrow,’ ” Detwiler said. “But we weathered the storm.”

And, when it came to end it, Harper delivered.

“I mean,” Manager Davey Johnson said, “he’s been unbelievable.”