A steady rain turned Nationals Park gloomy and delayed play for 2½ hours on Sunday as the Washington Nationals prepared to play the losing Mets. In any other season, by the time the game eventually began the “crowd” might have been just a few thousand die-hards. That’s what decades of disappointment naturally breeds.

This time, every part of the park, right up to the top sections, remained semi-filled as some fans left but the large majority of the 34,764 stayed. On the right field scoreboard they watched something unique (in D.C.): a live pennant race game in Atlanta between the Dodgers and the Braves that had direct impact on the Nats. With Atlanta arriving here Monday for three vital games, the potential swing of even a game — either way — had weight. Is momentum just “the next day’s pitcher?” Yes, if he’s good enough. Otherwise, momentum exists.

The Dodgers finished a 5-0 knockout of the Braves just seconds before the Nats started bludgeoning the Mets with an opposite-field, two-run homer by Danny Espinosastarting the scoring in the second inning. By the third inning, a Jayson Werth single, a Bryce Harper triple off the scoreboard and a Ryan Zimmerman RBI single gave the Nats a 4-0 lead.

A Harper homer into the right field bleachers, his second in two games, just dotted the “i” in win. Nearly five hours after scheduled game time, with the Nats ahead 5-1 and starter Gio Gonzalez out of the game, the park was more than half full to sing “Take Me Out to the Ball Game.” That’s normal in New York, Boston, St. Louis and many other cities. That’s how it’s supposed to be, if not even more exuberant. But it’s new here.

But then, what isn’t new in this season of altered states for Nats fans that are forced to figure how their maybe-a-wild-card-contender team is suddenly on a 100-win pace and hosting the Braves with a five-game division lead. This is a time when pennant race subtleties appear. For example, gaining a game on Sunday mattered plenty. If the Braves had gained a game over their weekend with the Dodgers, rather than losing a game as the Nats took two of three from New York, they’d have arrived in D.C. thinking: “Only three back. We could leave town tied for first place.”

What will they think now that they’re five back? Don’t know. Up to them. But it might be: We could leave town two, four, six or eight games behind. Those numbers “six or eight” sound like a lot, even in August. “Sure, we know Atlanta is coming in. But we keep concentrating on what is at hand — pay attention to today with an eye on tomorrow. That’s it,” Manager Davey Johnson said.

“It doesn’t hurt at all [to stretch the lead to five]. They know it full well,” Johnson said of the impact of the Nats gaining ground this weekend. “They need to prove themselves to themselves. And we always need to keep proving what we can do.”

The Nats’ mantra for months has been a simple one that hasn’t cracked yet. Late September or October may be a tougher test. But August shouldn’t stress their method. “We go out, have fun, play. At the end of the day, someone different helps us win,” Zimmerman said. “Or if we lose, we try to learn.”