Edwin Jackson missed a button as he pulled on his shirt late Saturday night. “Tired,” he said, shaking his head as he realized the mistake. “One of those days.” The fourth starter in the Washington Nationals’ rotation knew he did not have his best pitches — his “blow-away stuff,” as he called it — from the moment he set foot in the bullpen to warm up. Even on presumed off nights, as he and the rest off the deepest, best staff in the major leagues would show, the Nationals’ pitchers can conceal any ills.

The Nationals threw the full weight of their 12-armed monster at the Baltimore Orioles, evening the series with a 3-1 victory before a sellout crowd at Camden Yards.

Their offense, meanwhile, scraped together three runs. They determined Ryan Zimmerman, after going 0 for 4 and dropping his average to .218, is likely nearing another cortisone shot in his troubled shoulder and another stint on the disabled list. “If things keep going the way they’ve been going,” Zimmerman said, “we’re going to have to do something.”

The Nationals have received almost nothing on offense all season from Zimmerman, their cornerstone player. And still, they are in first place by 3½ games and loud music blared in their clubhouse Saturday night. They can thank their pitching staff, which has a 2.96 combined ERA. It was spearheaded Saturday by Jackson and Tyler Clippard, who converted his 12th save in 12 chance since he took over last month as the Nationals’ closer.

“Everybody is coming out every day to be the best pitcher that is on the field that day,” Jackson said. “That’s the mentality we all have when we take the field, to be the better pitcher that day. It’s just one those things when the team is rolling and everything is going good, everybody is positive and everybody is taking the field with a positive approach, and it’s just showing in the way we play right now.”

Clippard will only receive more chances. After the game, Manager Davey Johnson anointed Clippard as the closer even when Drew Storen returns from elbow surgery. Since May 22, when he first took over the ninth inning, Clippard has been robotic in his dominance. After a perfect ninth Saturday, including a strikeout of Adam Jones, he has allowed no runs and one hit and struck out 16 in 131 / 3 innings.

“Right now he’s my closer, and the way he’s going I can’t see going to somebody else,” Johnson said. “They’d have to show me up here probably in a setup role before they have the opportunity to close.”

The Nationals feel just as good about their rotation. Jackson lowered his ERA to 2.91. after allowing one run on four hits over 61 / 3 innings.

“That’s insane,” Clippard said. “It make us smile. I just can’t imagine what those other teams are thinking when Edwin is our fourth guy. It’s a joke.”

But Jackson knew early it would not be easy on Saturday, even though the Orioles’ starting lineup entered the game a 10 for 74 against him with 19 strikeouts. In the bullpen, he could not muster his usual velocity. In the early innings, he threw his fastball around 90 mph. Pitching coach Steve McCatty turned to Johnson and asked, “What’s wrong with him?”

The answer: Nothing. Jackson dominated the Orioles again. He cruised through the first three with 34 four pitches. At the end of the fourth, he had retired all 12 batters he faced.

“You just know you can’t really afford to get behind in the count too much,” Jackson said. “Days you don’t have your best stuff, you have to kind of come at them early and be able to throw off-speed for strikes as well.”

Adam LaRoche staked Jackson to a 3-0 lead, scoring one of two runs in the second inning when the Orioles bungled an infield single by Xavier Nady and then blasting a solo homer, his 14th, in the fourth off Orioles left-hander Wei-Yin Chen.

Jones led off the fifth inning with a hard groundball that skipped over Zimmerman’s glove to give the Orioles their first base runner on an error. The remaining drama lasted only more batter. Wilson Betemit rolled a clean single into right field, past LaRoche’s dive. With two outs, Nick Johnson walked to the load the bases. The only thing that mattered as No. 9 hitter Steve Pearce walked to the plate was protecting the three-run lead.

In his most important moment, Jackson turned to his best pitch. He threw Pearce his hellish version of a slider. The pitch zips in the mid-80s, and it looks like a fastball or change-up until and it breaks straight down, like a marble into a drain. Pearce swung and missed. Jackson threw him another slider, and all he could do was pop up to shortstop.

“Normally those are the days you pitch better,” Jackson said. “When you don’t feel the best and you’re not trying to overdo it, you down in the zone and pitching to contact and making them put the ball in play.”

The Orioles finally solved Jackson in the seventh — when he started feeling better, the ball started coming up. The first four hitters all hit the ball hard, including a solo homer by Jones. But he carried them deep into the game, allowing another easy passing of the baton from starter to reliever. Michael Gonzalez bailed out Jackson, and then Sean Burnett — who has a 1.04 ERA — and Clippard did the rest.

“It definitely helps,” Clippard said. “What they’re doing just gives the pitching staff more confidence. That’s the kind of team we have. We’re going to be in a lot of close ballgames. . . .Everyone knows that if we have a couple-run lead or whatever it is, with our staff, what our starters are doing and what we’re doing in the bullpen, I think it’s going to be tough for a lot of teams to come back.”