Washington Nationals right fielder Bryce Harper (34) rounds third base on his fifth inning solo home run. (John McDonnell/THE WASHINGTON POST)

Slump and all, Washington Nationals rookie Bryce Harper still etches a new line in history with every game. He has been through the highs of a rookie season — an all-star selection — and the lows — a slump that has challenged and frustrated him. He has been sent to the bench for days off to relax here and there, including Saturday, which doesn’t sit well with someone used to playing every single day.

In Sunday’s rain-delayed 5-2 win over the New York Mets at Nationals Park, Harper looked like the 19-year-old prodigy who captured the imagination of Nationals fans and the attention of those across baseball with a torrid first two months of his major league career. He smashed a run-scoring triple in the third inning and crushed a leadoff home run in fifth inning — each of which moved him into elite company among major league players his age.

“I think he was trying to send a message to me: ‘Don’t bench me. That’s what you’re missing,’ or something,” Nationals Manager Davey Johnson said of Harper. “He’s always swinging the bat good. I think sometimes he just gets a little frustrated and over-aggressive.”

With the biggest series of the season looming — a three-game set against the Atlanta Braves — the Nationals’ offense righted itself. The night before, baseball’s hottest offense over the season’s second half was shut out for only the fifth time this season. Another meager performance would put the offense on the type of streak it would want to avoid.

Behind Harper and Co., things looked better on Sunday. Danny Espinosa opened the scoring with a powerful two-run home run to the opposite field in the second inning. The team’s first three hitters — Jayson Werth, Ryan Zimmerman and Harper — finished a combined 6 for 12. Zimmerman narrowly missed adding a home run in the seventh inning when he slammed a double off the center field wall. They gave Nationals starter Gio Gonzalez enough offense to work through some shaky command and push the team’s lead over Braves in the National League East to five games.

“We really haven’t had that lineup in there but two or three days,” Johnson said. “I think it’ll start feeding off itself.”

Gonzalez labored at times after sitting through a 21 / 2-hour rain delay — a fact he refused to acknowledge was a factor — and too often fell behind in counts, leaving the ball high in the strike zone. But he wriggled out of jams on the strength of his hard fastball and late-breaking curveball to last 52 / 3 innings and earn his 16th win of the season, the most in Nationals history.

“I was just going out there and trying to keep us in the game as much as possible,” he said. “I was battling out there.”

Gonzalez worked around two singles in the first and another in the second. He allowed two base runners — a single and walk — in the third. He made a leadoff triple by Ronny Cedeno in the fourth inning disappear with two strikeouts and a flyout. His best shot at a clean inning came in the fifth when he struck out leadoff hitter Andres Torres and got a groundout from Justin Turner, but then he allowed a single to David Wright. He battled Scott Hairston, a scourge of left-handed pitchers, for eight pitches before ending the inning with a flyout to left field.

Gonzalez allowed a leadoff double to Ike Davis in the sixth inning, prompting Johnson to call for Craig Stammen to warm up in the bullpen. Gonzalez nearly escaped again but a two-out walk to Kelly Shoppach ended his day after he allowed only one run but threw 110 pitches. Stammen finished the inning and another, and the rest of the bullpen let the Nationals’ lead stand.

Since the all-star break, Harper has been one of among the easiest outs in baseball. Pitchers have challenged him with breaking balls, pitches off the plate and gotten some generous calls, too. Johnson, now dealing with a healthy and complete lineup, could afford to give Harper a mental break.

After Harper missed the cutoff man on a throw from the outfield in Friday’s game, Johnson told the rookie he wouldn’t start the following day. Harper, in Johnson’s account, wanted to strangle him. He might not have understood what Johnson was doing, the manager said, but it was needed.

“I’m never satisfied with my work,” Harper said in a muted tone following the win. “I’m never satisfied with anything I do. We’re in first place and that’s the biggest thing I care about. My numbers could be really crappy but we’re in first place. That’s all that matters.”

Harper’s smile and excitement were evident on the field Sunday. After he slid into third base with a triple that gave the Nationals a 3-0 lead, he jumped up, slapped his hands together and pointed to the dugout. Following his 12th home run of the season, which made it 5-0, he darted around the bases at breakneck speed and then celebrated with teammates in the dugout. In the series, Harper went 4 for 8 with two home runs and five RBI.

With his blast on Sunday, Harper became the fifth teenager in major league history to hit at least 12 home runs in a season, joining Mel Ott, Tony Conigliaro, Ken Griffey Jr. and Mickey Mantle.

With his sixth triple of the season, he became the first teenager to notch that many since Buddy Lewis did in 1936 for the Senators. He is the first teenager in major league history with six triples and 12 home runs.

“He crushed that ball today,” Johnson said. “I haven’t seen one hit that hard.”

Added Espinosa: “As long as he doesn’t press and as long as he’s not trying to force things to happen, he’ll be fine. Everyone sees the amount of talent he has.”