The maddening first half of the Washington Nationals’ regular season ended Sunday afternoon with a pulverizing reminder of how good they can be, their sine-wave tendencies on full display. The day after a journeyman deadened their bats, the Nationals thrashed the New York Mets’ prized pitching prospect. One of their three aces carved seven scoreless innings as their defense atoned for three Saturday errors with flawless fielding. They erupted, not bothering to wait a day for the return of Bryce Harper. In a 13-2 demolition at Citi Field, the Nationals received their largest offensive output of the season as Gio Gonzalez continued his recent brilliance. The Nationals thumped high-profile rookie Zack Wheeler for 42 / 3 innings and pounded the Mets’ bullpen so severely they sent backup catcher Anthony Recker to the mound in the ninth inning. Ian Desmond launched one of Recker’s batting-practice fastballs to the upper deck in left field, completing the Nationals’ four-homer, 10-extra-base-hit onslaught. The walloping nudged the Nationals’ record to 41-40 as they hit the halfway mark, back above .500 — again — and 6½ games behind the first-place Atlanta Braves. The preseason favorites have taken a bumpy path, but they have won seven of 11 games, and they do not mind the need for an uphill climb. “I like where we’re at,” right fielder Jayson Werth said. “We’re in good stalking position. Of course it’s easier just to go wire to wire and not be put to the test. You’re almost better heading into the playoffs having to play a bunch of meaningful games in September, games you got to win, coming from behind. I think that sets you up better for the postseason.” The Nationals have proved a single victory will not catapult them out of their season-long muddle. But one player might. The Nationals have played without Harper since May 26, a span of 31 games during which they have gone 15-16. On Monday at Nationals Park, he is expected to rejoin them. The Nationals have been average or worse in most every circumstance except one. With Harper in the starting lineup, they are 25-18, a 94-win pace over a full season. “Just his presence, I think it gets everybody going,” center fielder Denard Span said. On Sunday, they drilled the Mets while Harper watched from home. Every starting position player chipped in at least one of the Nationals’ 13 hits and one RBI. Desmond, Werth, Adam LaRoche and Kurt Suzuki smashed home runs, and Span went 3 for 5 with two doubles. “It’s a boost, especially since we haven’t seen it a whole lot this year,” LaRoche said. “We know we’ve got the lineup that can go out any given day and put up 15 hits and 10 runs, and it just hasn’t happened. This isn’t going to happen every day, obviously, but it should a lot more often than it has.” Manager Davey Johnson has pleaded with Nationals batters all season to be more aggressive hunting fastballs. Against Wheeler, Span said, the Nationals planned to spit on off-speed pitches until he proved he could throw them for strikes. When he could not, the Nationals feasted on fastballs. They belted five of their six hits off Wheeler against his fastball, which sat in the mid-90s, including two home runs and two doubles.