Bryce Harper is tagged out after swinging through a ball in the dirt to end the Nationals’ loss to the Cubs. (Jonathan Daniel/Getty Images)

The Wrigley Field faithful, nearly 42,000 people on a sun-kissed Saturday afternoon, rose to their feet in unison as the Washington Nationals’ No. 3 hitter began his walk from the on-deck circle. They all recognized the moment’s magnitude. They were watching a marquee matchup they will see again in early October — if the standings hold over the next two months — boil down to one heavyweight battle: all-star slugger Bryce Harper against all-star closer Wade Davis in the ninth inning of a three-run game with two on and two outs.

Last year, the Chicago Cubs refused to pitch to Harper at Wrigley, walking him 13 times and hitting him with a pitch another time over a four-game series. But they are pitching to the slugger this weekend, and he had already taken advantage way back in the first inning, when he smashed a John Lackey fastball into the video board down the right field line for a home run. The Cubs’ strategy remained unchanged in the ninth, and Harper, representing the tying run, was looking for a second one.

“I’m just trying to hit a homer,” Harper said, “tie it up a little bit and go extras and roll the dice.”

That did not happen, though, because Harper fouled off the two strikes Davis threw him before swinging through a curveball down and out of the zone. It concluded a 7-4 Nationals loss.

“We thought we had a great chance of snatching that game away from them,” Nationals Manager Dusty Baker said.

A comeback was required because Washington starter Edwin Jackson floundered in the first inning before reversing course, and a couple of incumbent members of the Nationals’ overhauled bullpen spoiled Jackson’s domination from innings two through five.

The Cubs immediately capitalized after Jackson departed with Chicago leading 4-3. Willson Contreras crushed Matt Grace’s third pitch of the day for a two-run home run in the sixth inning, which concluded with Nationals catcher Matt Wieters getting ejected by home plate umpire Chad Whitson for critiquing Whitson’s strike zone.

“He pretty much already gave me a warning to quit arguing balls and strikes,” Wieters said. “I was more arguing that they had a different strike zone than we had. . . . Our left-handed hitters were getting some pitches called that were off the plate to them, and we had a couple pitches that last inning that we didn’t get that were really close, if not strikes.”

Chicago then manufactured a run in the seventh against Matt Albers for additional insurance. Four Cubs relievers, meanwhile, combined to yield one unearned run over four innings as Chicago (58-51) snapped a three-game losing streak and evened the season series with the Nationals (64-44) at three.

It had been nearly three years since Jackson made his last start on the North Side as a maligned piece of the Cubs’ rotation. Saturday represented an opportunity, even if he wouldn’t say it, to exact some revenge on an organization whose president blatantly remarked that signing Jackson was “a mistake” — while Jackson was still on the Cubs’ roster with two years remaining on his contract.

“That may be hyped up more by other people,” Jackson said. “For me, it’s another team. The objective is the same: go out, give us a chance to win.”

For four innings Saturday, Jackson displayed the talent that compelled Theo Epstein to give him a four-year, $52 million deal five winters ago after Jackson spent the 2012 campaign in Washington. He was overwhelming with a fastball in the mid-90s, his off-speed stuff still boasting so much life a month shy of his 34th birthday. He allowed one hit, struck out seven and didn’t walk a batter in those four scoreless frames. But the stretch arrived too late, after a disastrous four-run first inning.

The Cubs welcomed Jackson back with back-to-back doubles — from Jon Jay and Kris Bryant — to tie the game two batters into the bottom of the first. Two batters later, Contreras hit a perfectly placed dribbler down the third base line. By the time Jackson picked up the baseball, Bryant was crossing home plate and Contreras was a couple steps from reaching first. Jackson ate it. 2-1.

Two batters after that, Alex Avila provided the big blow: a two-run home run, just beyond the wall and into the basket in left-center field, to make it 4-1. It was Avila’s first home run since the Cubs acquired him from the Detroit Tigers on Monday.

Jackson didn’t yield another run as he completed his fourth start as a National this season with eight strikeouts and no walks. But that 31-pitch first inning eventually forced Baker to remove him after five innings with his pitch count at 101.

The Cubs led 7-4 entering the ninth, and Davis got pinch hitter Howie Kendrick to ground out to lead off the inning before he encountered a bout of wildness. He issued consecutive one-out walks to Jose Lobaton, a .157 hitter, and Brian Goodwin, a .240 hitter, but recovered to strike out Wilmer Difo and set the stage for Harper.

Four tense pitches and one smoked line drive foul ball later, Harper struck out swinging, igniting another rendition of “Go, Cubs, Go” for the elated spectators.

“I just always believe that we have a chance to come back,” Baker said. “Especially when we got the tying run at the plate in Bryce.”