Catcher Wilson Ramos is helpless to prevent San Diego’s Cameron Maybin (top) and Orlando Hudson from crossing the plate with all the offense the Padres would need to snap the Nationals’ winning streak at four. (Denis Poroy/Getty Images)

Tyler Clippard had already squirmed out of another teeth-clenching jam when he emerged for the eighth inning Thursday night. He had already experienced the kind of strain that had become his new norm this season. He would labor for more outs as a victory slipped away, a capable bullpen of fresh relievers just waiting for the call from Manager Davey Johnson.

But Johnson continued to maintain a singular thought: There was no one he would rather have on the mound than Clippard.

The Washington Nationals asked Clippard for one more escape act, and for once he could not deliver. After Johnson stuck with Clippard for a second inning, the San Diego Padres stunned Clippard with two runs in the eighth and beat the Nationals, 2-1, to prevent Washington from completing a sweep before heading into Dodger Stadium for a showdown between the National League’s best two records.

Edwin Jackson dueled Padres starter Edinson Volquez for much of the night, the only damage off either pitcher coming when Jayson Werth drilled a solo home run in the seventh inning to give the Nationals (14-5) a 1-0 lead. Jackson’s 6 2 / 3 scoreless innings made the Nationals the first team since 1900 to receive scoreless starts in eight of their first 19 games.

The Nationals fell to 6-3 in one-run games. They had come to believe they would find a way to snuff every threat and generate a clutch hit when needed, no matter what happened during the first eight innings. Thursday night, as they managed only four hits, the Nationals could not do it again.

“We expect to win those close ballgames,” Clippard said. “It was a pitchers’ duel, and we got the situation we wanted to be in with the lead late in the game. That’s the kind of team we are. We understand that. We’re all confident in close games. We’ve won them all year.”

This night would unfold differently. Clippard relieved Jackson in the seventh inning with two men on and two outs. He promptly walked Chase Headley, a sign of the command difficulty that led to him throwing 26 pitches Wednesday night.

Clippard has been gutsy this year, but not his dominant self. Over the past two years, batters missed his pitches 33 percent of the time they swung. This year, the number has dropped to 21 percent.

“They know him a little better,” pitching coach Steve McCatty said. “They’re putting better wood on the ball, fouling pitches off. They’re doing reports on him. I don’t think he’s not as good as last year. They’re just making contact a little more. They study these guys.”

But he is still the Nationals’ all-star set-up man, the pitcher who punched up a 1.83 ERA and struck out 10.6 batters per nine innings last year, the pitcher Johnson turns to in the toughest jams. After Nick Hundley smashed two change-ups foul, Clippard blew a 2-2, 94-mph fastball past him with the bases loaded. He pumped his fist on the mound, an exclamation point he had used in recent clutch escapes.

“You definitely want him with the ball at the end,” Werth said.

Clippard had yet to pitch across multiple innings this season, but he told Johnson and McCatty he felt great. Even with Ryan Mattheus and Sean Burnett fresh, Johnson stuck with Clippard for the eighth. He wanted the ball in Clippard’s hands.

Clippard has said high pitch counts do not wear him down, but pitching across multiple innings tests his stamina. He retired the first batter he faced, but as his pitch count climbed beyond 20, the Padres did their damage.

Orlando Hudson drew a walk. Cameron Maybin reached on a bunt base hit. No one stirred in the Nationals’ bullpen. Padres Manager Bud Black chose as a pinch-hitter left-hander Mark Kotsay, who on Tuesday worked a 12-pitch at-bat against Clippard.

“In hindsight, I could have had Burnett ready,” Johnson said. “But that’s Tyler Clippard. It’s hard for me to think about hooking him.”

Tuesday, Clippard had won his won his confrontation with Kotsay by feeding him changeups. Kotsay fouled off nine pitches and took two balls before he finally popped up to short. Clippard, much to his regret, abandoned the gameplan Thursday and threw Kotsay nothing but fastballs.

“I wish I had the answer,” Clippard said. “Kind of a mental lapse, I guess. I got him out the other night in a long at-bat with changeups. I felt like he might have been sitting on that pitch. It was more or less second-guessing myself, which is something you don’t need to do out there. I’ve said it all along, I’ve got to stick to my strengths.”

On the fourth pitch, Kotsay roped a double to the wall in right-center. Both runners scored, and the Nationals trailed with three outs left. Only after Clippard struck out Andy Parrino did Johnson turn to Burnett. Some Nationals believed home plate umpire Scott Barry had not given Clippard the low strike, but he blamed only himself.

“It was just poorly executed, and the pitch selection wasn’t good, either,” Clippard said. “It was kind of frustrating on my part.”

The Nationals also ran into the reality of scoring only 3.63 runs per game. They have thrived in close games, but they cannot live forever on such small margins of error. “If we keep playing these close games, somebody’s going to come up with a big hit,” first baseman Adam LaRoche said. “It happened tonight.”

Their dearth of offense squandered Jackson’s outstanding performance. Jackson allowed six hits and three walks while striking out six, and his slider was the difference. Rather than whipping across the plate like most sliders, Jackson’s dives straight down. He typically throws it hard, about 85 mph, but he was throwing it in the high 80s and the Padres could not figure it out. Jackson threw sliders on almost one-third of his pitches, and he turned to the pitch in his toughest spots.

“In hitter’s counts, I tried to go soft instead of hard,” Jackson said. “I got in some jams, and I was able to wiggle my way out of them.”

In the fourth inning of a scoreless game, Hundley led off with a single to center. Yonder Alonso followed with a missile off the base of the right field wall, putting two runners in scoring position with no outs. Jackson needed three outs, perhaps two strikeouts, to keep the score tied.

Hudson fouled off a first-pitch fastball, and then Jackson threw him two sliders, 87 and 89 mph, and Hudson swung over both. One out.

Jackson intentionally walked Maybin to load the bases, bringing to the plate Jason Bartlett with a double play possible. Jackson dismissed any pitch but his slider. He fired three of them, at 89, 87 and 88 mph. Bartlett swung at all three and found only air. Out No. 2.

Jackson had set aside the hard part, needing only to retire Volquez, the pitcher, to escape the jam. Volquez chopped to shortstop Ian Desmond, who made a charging play and running throw to first. Out No. 3. Jackson had given the Nationals’ offense another chance to score first.

They finally took it in the seventh. But Johnson asked Clippard, and only Clippard to hold another lead. For once, he could not.

“He’s done such a good job for us,” Werth said. “You got to cut him some slack at some point. He’s a competitor and he wants the ball. I don’t think it’s the easiest thing to come in, sit for a half inning and come back out.”