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WHAT IT MEANS: Jordany Valdespin did it again. Only this time, the script turned out more like a repeat of a June 5 loss to the Nats than a reprise of the rookie's Philadelphia heroics.

For the second time this season, Valdespin delivered a pinch-hit, three-run homer in the ninth off a top NL East closer.

Valdespin -- who beat Jonathan Papelbon in Philly on May 7 -- batted as a pinch hitter for Jason Bay in the ninth (more on that later) and deposited a pitch over the right-center wall against Tyler Clippard with one out to give the Mets a 3-2 lead.

Bobby Parnell then blew his second straight save opportunity in Frank Francisco’s absence, failing to protect the one-run lead in the bottom half. Danny Espinosa delivered a game-tying RBI single with the Nats down to their last strike to force extra innings.

Josh Thole restored a one-run lead in the top of the 10th on an RBI double. But Tim Byrdak also failed to close it out. He surrendered a game-tying triple to Bryce Harper. Byrdak departed with two outs and the bases loaded. Pedro Beato then uncorked a game-ending wild pitch as the Mets lost 5-4 in 10 innings.

It mirrored a loss to the Nats in the opener to a series here last month, when the Mets squandered leads in the eighth, 10th and 12th innings and lost 7-6.

So it turned out to be the Mets' fourth consecutive loss to open the second half, and fifth straight overall.

The Mets (46-44) dropped a season-high seven games behind the first-place Nats in the division standings. They slipped to two games over .500 for the first time since they were 22-20 on May 21.

Last season, the Mets lost on a walk-off balk in Atlanta by D.J. Carrasco on June 16.

OFF THE HOOK: Before the ninth, Jon Niese looked like he was headed for a tough-luck loss.

Niese had retired 12 straight Nationals before a one-out homer in the fifth by southpaw-slayer Tyler Moore broke a scoreless tie. Josh Edgin then surrendered an eighth-inning run as Washington took a 2-0 lead into the final inning.

Niese departed for a pinch hitter in the bottom of the seventh, having allowed one run on three hits. He received a no-decision once Valdespin went deep.

BAY WATCH: Bay went 0-for-3 in his return from the disabled list and was front and center in the offensive frustrations. His at-bats were so futile Terry Collins lifted him for Valdespin in the ninth, and it's hard to force the manager justifying regular playing time going forward.

Bay grounded into a 6-4-3 double play in his first at-bat, as the Mets failed to score despite three singles in the second inning.

Two innings later, Bay was retired on a comebacker to opposing starter Ross Detwiler.

In the seventh, Bay flied out to center fielder Bryce Harper -- too shallow for David Wright to tag up from third base.

Bay was not the only one to let the Mets down in the seventh.

Wright, who had flied out to the right-field wall in his previous at-bat, this time opened the seventh with a double beyond the reach of right fielder Michael Morse. Wright advanced to third on a wild pitch, but was stranded there when Scott Hairston struck out, Bay had the shallow flyout and Ike Davis grounded out to second base.

The Mets fell to 12-21 this season in games in which the opposition starts a left-handed pitcher. That is particularly alarming considering GM Sandy Alderson reiterated pregame that Bay is likely the Mets' trading-deadline righty-bat addition.

Alderson said his priority is acquiring bullpen help.

Of course, unless help arrives imminently, the Mets' swoon to open the second half may make it illogical to acquire a reliever, since the season could be lost.

It will be interesting to see whether Collins uses Bay against right-handed starter Jordan Zimmermann on Wednesday. (The bet is no.)

With Kirk Nieuwenhuis and Lucas Duda likely starting at two outfield positions now that Duda has received a cortisone shot in his left hamstring, Collins logically could relegate Bay to platoon player and wait until Thursday's matchup with southpaw Gio Gonzalez to start him again. Valdespin or Andres Torres could fill the third outfield spot Wednesday.

Even former hitting coach Howard Johnson, watching elsewhere on TV, tweeted: "With one swing Valdespin might have just saved the Mets season and signaled the end of Jason Bay in NY."

WHAT’S NEXT: Chris Young (2-3, 4.28 ERA), who started the second-half opener in Atlanta, opposes Zimmermann (6-6, 2.48) at 7:05 p.m. ET Wednesday.