Jayson Werth, left, puts on the brakes after rounding first base after singling in the third inning. Mets first baseman Ike Davis heads back to the bag. (John McDonnell/THE WASHINGTON POST)

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.”

This week’s pitching matchups are anything but accidental. The Nats open with their other two aces, MLB ERA leader Jordan Zimmermann and Stephen Strasburg, the majors’ leader in strikeouts per inning. The Braves counter with perennial ace Tim Hudson, whose slider has, in previous years, made the Nats’ right-handed hitters look ridiculous. He’s followed by trade-deadline addition Paul Malholm, a speed-changing lefty having his best season and coming off one run in his last 16 innings.

For the finale, Johnson has flipped Edwin Jackson and Ross Detwiler in his rotation “to separate” his southpaws (Gonzalez and Detwiler). Oh, and getting Detwiler against the Braves, who are vulnerable to lefties, probably had nothing to do with it.

Detwiler may also be the Nats’ biggest X-factor for the postseason. Since the All-Star Game, he’s gone to almost 90 percent two-seam or four-seam fastballs. In those games, he’s raised his fastball velocity, the true “easy gas” of a 6-foot-5 crossfire lefty, into the top 15 in baseball with a 2.91 ERA. If he can thrive as a pure power pitcher with an exceptional sinker, that style tends to play even better against tight hitters in October. Or will the style tweak flop? The start against Atlanta may be a playoff-preview test.

“We’ve been telling Det that 95 [mph] is not easy to hit. It’s the same thing Davey tells [Strasburg]. As a hitter, I know that nothing bothers you more than a guy who attacks you, and keeps on attacking, with a good moving fastball,” Zimmerman said. “It makes hitters defensive. Aramis Ramirez got to third base and said to me, ‘Do any of your guys throw less than 95?’ ” Actually, the whole rotation tops at 96 to 99 mph.

Pennant race dynamics take strange forms. The Nats may have more opportunity — and necessity — in this week’s Atlanta series than they suspect. The Braves are about to begin what may be the toughest 10-day trip of their season, including seven on the West Coast, with their visit here. But after that, they play the patsies from pennant-race heaven.

The Nats may need a significant lead by Aug. 31 because a strong Braves team has one of the easiest late-season schedules you’ll ever see. After that date, they play 16 straight games against teams that are well under .500, then host the Nats for three in Atlanta, followed by 12 more in a row with losers (before closing the season with three at Pittsburgh). That 31-game run against bad teams, plus one last shot at the Nats, almost insures that the Braves aren’t going away.

“It’s not always so easy to play the teams that are out of it. I’ve been there,” Johnson said. “They have absolutely no pressure and love to spoil it.”

Washington also has an easy schedule, on paper, and 24 more home games. But the Nats’ foes are not as lame as those the Braves play — if, a huge “if,” they survive their 10-day trip that begins on South Capitol Street.

If the Nats want somebody to spoil the Braves’ fun, if they want to start Atlanta on its last long tough West Coast trip on a negative note, they’d be wise to do it themselves.

For previous columns by Thomas Boswell, visit washingtonpost.com/boswell.