ST. LOUIS — Adam Eaton walked to the plate Saturday night looking for his first hit of the day. He’d had three tough at-bats early, but now, in the late innings, he had the chance to come through big. He had the chance give the Washington Nationals an insurance run or two to protect a gem by the starting pitcher.

It was the same exact situation for the second night in a row, and he delivered again with a one-out, extra-base hit.

Eaton’s two-run double down the right field line in the eighth inning provided the Nationals some crucial breathing room in a 3-1 win over the St. Louis Cardinals, giving them a 2-0 lead in this best-of-seven series. It proved to be the difference after Michael A. Taylor misplayed a line drive to center a half-inning later, allowing what could have been the tying run to score. Instead, the cushion afforded by Eaton held up and helped expand the Nationals’ lead — putting them two wins away from their first World Series appearance.

“Biggest hit of the year so far,” shortstop Trea Turner said

The pivotal double came one day after he delivered the other most important insurance run of the season. In Friday night’s Game 1, Eaton smacked a triple to left-center field and later scored on a Howie Kendrick single to give the Nationals breathing room in a 2-0 win. The hit in Game 2 somehow seemed even more surprising because the Cardinals’ starter, Adam Wainwright, had befuddled Eaton all afternoon.

The right-hander mixed pitches, locations and speeds against the veteran outfielder. Eaton flied out to left field in the first inning on Wainwright’s signature 12-to-6 curveball. He grounded out meekly in the third on a sinker off the plate. He struck out looking in the sixth when Wainwright started outside with a curveball and a fastball and then came back in with another sinker.

The shadows in Busch Stadium for the afternoon game didn’t help. When the ball leaves the pitcher’s hand, the hitter looks at the seams, and the direction of the spin usually tells him what type of pitch it is. Shadows sometimes make the ball appear gray and cloud the seams. Eaton pointed out that Kolten Wong, the Cardinals’ second baseman, check-swung twice and barreled up what became two groundouts.

“You just saw some really bad swings and bad counts [for everyone],” Eaton said.

Wainwright had allowed four hits and one run in seven innings when he faced Eaton again in the eighth. Matt Adams and Turner had singled, so Eaton had two on and one out. It seemed possible that, with traffic and Wainwright nearing 100 pitches, Manager Mike Shildt might go to left-hander Andrew Miller. But Shildt stayed in the dugout.

Eaton wasn’t contemplating whether he would face Miller or Wainwright. He believes considering what a manager might do distracts from the focus of getting a hit against the man on the mound. He needed to concentrate because Wainwright and catcher Yadier Molina had confused him all day, keeping him “really, really off balance.”

The sun had gone down, meaning he was seeing the ball all the way to the plate for the first time. Eaton did what he has done best for this team all season and wore down the pitcher. He laid off ­curveballs that just missed up and away and coaxed Wainwright back inside. He fouled off a fastball that would’ve been strike three and got the count to 3-2.

Eaton thought whatever Wainwright threw, he needed to throw it for a strike. He decided it should be a fastball, but then he stopped himself. He thought of George Costanza, the sitcom character from “Seinfeld.” The short, stocky, ­follicly challenged man decided at one point in the show to disregard his instincts and do the opposite. Eaton channeled his inner Costanza: He forgot the fastball and geared up for the curve. He felt an unmatched conviction “more so than any other pitch — or any other at-bat that I’ve had.”

“That’s what happened,” Eaton said. “George was right.”

Eaton poked the curveball just fair inside first base and sprinted to second base as Adams and Turner scored. Shildt marched out of the dugout to replace Wainwright. Adam had knocked in Adams as he knocked Adam out.