WASHINGTON -- Heading into their current stretch of 14 consecutive games against the Nationals and Marlins, the Mets had already shaved away any margin for error they once possessed in the National League East race. That didn’t mean their realistic path to the postseason had vanished; it just meant that to follow it, the Mets would need to win nearly every game they played against the division’s bottom-feeders before advancing to a more difficult stretch of schedule later this month.

In other words, the Mets needed to be nearly perfect. They haven’t been.

• Box score

And while three wins in five games may be just fine under normal circumstances, the Mets could ill afford their walk-off 4-3 loss to the Nationals on Monday in Washington -- particularly considering they led for nearly the entire afternoon.

Edwin Díaz’s second consecutive blown save -- his 17th in three seasons since joining New York -- resulted in this latest defeat for the Mets, who dropped back to .500 (69-69).

“You do start standing watch,” catcher James McCann said. “You do start seeing what other teams are doing, and that can be a dangerous situation if you’re worried about what other teams are doing, if you’re worried about what else is going around in the league instead of just taking care of your business.”

Because the Braves (72-64) were idle on Monday, the Mets did not need to concern themselves too much with the out-of-town scoreboard. Of greater issue was their own game, which they were ahead in late thanks to Pete Alonso’s go-ahead solo homer -- his 30th of the season -- in the sixth. The Mets survived a significant scare in the bottom of the eighth when Javier Báez made a Gold Glove-caliber play to prevent the tying run from scoring, but New York could not add any additional offense of its own.

That led to Díaz pitching in a one-run game, fresh off his most recent blown save in Friday night’s 6-2 win. Díaz immediately elicited concern Monday when he walked two of the first three batters he faced. Up came pinch-hitter Andrew Stevenson, who had knocked a game-tying two-run homer in the seventh on Saturday afternoon.

This time, all Stevenson needed to do was pull a Díaz slider into right field for a game-tying single. Although the Mets challenged the play, contending that Alcides Escobar did not touch home plate, a replay review was inconclusive enough for the run to stand.

“I’ll go off of what our [replay] guys said here,” McCann said. “There’s a reason why we challenged the play. I don’t think, in that moment, we’re going to search for something that’s not there.”

Moments later, Carter Kieboom hit a ground ball up the middle to end things. With Stevenson running on the pitch and Francisco Lindor ranging to his left, there was nothing anyone on the Mets’ infield could do to stop the inevitable.

“I didn’t command my pitches the way I wanted to,” Díaz said. “I couldn’t command my slider low in the zone, and I paid [for it] right there. I got hit.”

Afterward, McCann still defined the series win as a “successful weekend” for the Mets, who did gain a game of ground in the NL East. Manager Luis Rojas (who offered Díaz a vote of confidence) noted that his team is “closing the gap.” And while it’s true that gaining a game on the Braves every four or five days would result in a playoff berth for the Mets, the reality is that the schedule is about to turn against them. After three games in Miami this week, the Mets will have to play the Yankees, Cardinals, Phillies, Red Sox and Brewers in succession -- all of them playoff contenders. The Braves, meanwhile, have a chance to fatten up against the Nationals, Marlins, Rockies and D-backs.

The Mets’ goal is to have their final three games in Atlanta from Oct. 1-3 be meaningful; all they need to do is to enter that series no worse than three games back of the Braves.

But they will need to play more consistent baseball to make that a reality.

“The key is putting blinders on,” McCann said. “It’s definitely down to a sprint. It’s not a marathon anymore. You’ve got to put blinders on and not let outside noise, not let a game like today snowball into anything else.”