"It doesn't help," Mets manager Terry Collins said of the rain delay. "It didn't help Gio, either. It's something you don't want to have to go through, but it's part of what you've got to do. It's how you've got to plan.
"I know Hef was ready to pitch, he just made some mistakes in the middle of the plate."
Hefner denied feeling any consequences from the rain delay.
Gio Gonzalez started for the Nats and threw 5 2/3 innings, allowing just one run on seven hits. The left-hander moved the Nats to 75-46, five games ahead of the Braves in the NL East, with his 16th win of the season. Gonzalez is tied with Cincinnati's Johnny Cueto and Tampa Bay's David Price for the most wins in the Majors. The Mets, meanwhile, fell to 57-64, 18 games behind the Nats.
"That wasn't pretty, but it was a win," Nationals manager Davey Johnson said. "Gio was having the ball up all day long. He was a little wild ... I didn't plan to go get him in the [sixth] inning."
Two batters after Michael Morse singled to lead off the second inning, Espinosa launched a 75-mph curveball from Hefner to the opposite field and into the Mets' bullpen in left-center.
"I definitely knew I hit the ball hard," Espinosa said. "I wasn't sure if it was going to carry it out, but I thought it would at least hit off the wall for a double."
In the third, Hefner allowed another pair of runs when Jayson Werth led off with a single, Harper tripled him home, and Ryan Zimmerman added an RBI single.
Harper led off the fifth with his 12th homer of the season, off the second-deck scoreboard in right-center. The 19-year-old outfielder continued to show signs of emerging from his second-half slump, finishing 2-for-4 with two runs scored and two RBIs to boost his average back up to .251.
"In my bullpen [session] before the game, I felt like I had good stuff," Hefner said. "It just came down to execution, and I didn't execute. Throughout the course of the game, at least in my experience, you make mistakes and sometimes you get away with them, and sometimes you don't. Today, it seemed like I didn't get away with any."
The Mets' first run came in the sixth, which Ike Davis led off with a double to right field. After advancing to third on a Ronny Cedeno fly ball, Davis scored when Jason Bay bounced a grounder to short. New York added another run in the eighth, when Cedeno knocked a one-out double to left field off Nats reliever Mike Gonzalez and scored two batters later on an infield single by Kelly Shoppach. Daniel Murphy pinch-hit for Mets reliever Manny Acosta, but struck out looking on three pitches and was ejected thereafter by home-plate umpire Jordan Baker.
The Mets had threatened in the seventh after a pair of one-out singles from Justin Turner and David Wright. But Scott Hairston, 5-for-8 in the series, bounced into a 5-4-3 double play to kill the inning. In total, the Mets left 10 runners on base and went just 1-for-11 with runners in scoring position. Over their last 16 games, the Mets are batting .194 with RISP.
"You've got to get key hits," Collins said. "We sat here the entire first half [of the season] and talked about how many key hits we didn't get with two outs. We're not getting them, and that's a big difference."
Acosta pitched a perfect 1 1/3 innings with two strikes, while Josh Edgin allowed only a hit in two-thirds of the seventh. Bobby Parnell tossed a scoreless eighth for the Mets.
The loss dropped the Mets to 4-7 in rubber games and marked their ninth series loss in 11 tries in the season's second half.