Mets starter Dillon Gee held the Nationals to one run over six innings of work Saturday as New York spoiled the debut of Taylor Jordan. (Jim McIsaac/GETTY IMAGES)

All season the Washington Nationals have instantly flushed any optimism they managed to spark, unable to string together victories or capitalize on momentum. The pattern veered to the extreme Saturday afternoon, when the Nationals followed their self-described biggest win of the season with one of their shabbiest losses. A stirring comeback Friday night became a diversion on the way back to the familiar territory of a .500 record.

On the day before their season reaches the halfway point, the Nationals’ 5-1 loss to the New York Mets at Citi Field dropped them to 40-40, break-even again. The Nationals’ sloppy defense and nonexistent offense marooned 24-year-old starter Taylor Jordan, who made his major league debut and found play behind him fit for Class AA.

“You can’t explain it,” Manager Davey Johnson said. “It was an uplifting game last night, and today was a downer.”

Jordan, a quickly ascending call-up from Harrisburg, validated the Nationals’ faith in him over 41 / 3 innings, but the Nationals’ defense obscured the quality of his outing. Third baseman Ryan Zimmerman committed two fielding errors and Ian Desmond made a throwing error, all of which contributed to two unearned runs. Zimmerman’s 13 errors for the season are among the most in the majors, and the Nationals’ 59 errors pace the National League.

“He pitched his butt off,” Desmond said. “We didn’t really help him out too much.”

Meanwhile, the Nationals continued their odd capitulation to Mets right-handed starter Dillon Gee, who might be plying his trade in Class AAA Las Vegas if not for his bullying of Washington. The Nationals’ lone run in six innings against Gee came on Kurt Suzuki’s RBI single in the fourth. Against the rest of the league this year, Gee owns a 5.55 ERA over 13 starts. In three starts against the Nationals, he has allowed two runs over 182 / 3 innings.

“I think it’s more us,” Desmond said. “No offense, this is no knock on him. But I think it’s more us, maybe being a little too aggressive or not aggressive enough.”

Two months ago, Jordan was a Class A pitcher. Saturday morning, the right-hander sat in a chair in front of his locker inside the plush visiting clubhouse at Citi Field, smiling like he had just found a $20 bill on the ground. He underwent Tommy John surgery in 2011, and this spring training the Nationals did not bother inviting him to major league camp. After a promotion to Class AA, where he punched up a 0.83 ERA in nine games, he had made the major league roster.

“This exceeded even my high goals,” Jordan said. “So it’s unreal.”

Jordan stands 6 feet 3 and an athletic 190 pounds, “a nice pitcher’s frame,” Johnson said. As he delivered the ball, Jordan’s arm moved as if it contained an extra joint, loose and free, twisting behind his head and whipping quickly forward. He repeats the motion for all of his pitches, which adds more deception. His fastball reached 95 mph, and he mixed in change-ups and sliders.

“He just went right at them,” Zimmerman said. “I like guys that go right at people and make them put the ball in play.”

The hallmarks of a debut attended the afternoon. After Jordan’s first pitch, Suzuki rolled the ball into the Nationals’ dugout for posterity. Jordan had distributed 20 tickets to parents, relatives and friends. Before the game, Jordan asked Johnson if he had the game ball. After he stopped laughing, Johnson told Jordan to try pitching coach Steve McCatty.

“I wasn’t as nervous as I thought I was going to be,” Jordan said.

Jordan navigated through three scoreless innings in only 44 pitches. His sinker made the Mets’ bats sound hollow, and the weak contact made up for striking out only one batter — Gee — all outing.

In the fourth, Jordan’s new teammates began to betray him. After the Nationals had taken the lead, Marlon Byrd led off with a smash right at Zimmerman. The ball “just kind of stayed down,” Zimmerman said. It skipped through his legs and deflected of his foot.

After a walk and fielder’s choice, Jordan surrendered an RBI single to John Buck, with Byrd scoring the tying run. Jordan escaped further damage, but he threw more pitches than necessary after a pop fly dropped between Desmond and left fielder Roger Bernadina.

In the fifth, Byrd came up again with Daniel Murphy on first and one out. Byrd whacked another hot shot at Zimmerman. Zimmerman focused on staying down, conscious off his last misplay, but this time the ball bounded off the lip of the dirt and hopped high. It bounced off Zimmerman’s shoulder, another error.

“They’re plays that I should make, but just unfortunate, kind of tough plays,” Zimmerman said.

Desmond corralled the booted ball with his bare hand, and a simple error, partially due to bad luck, turned into a circus. From shallow left, he fired to third base, trying to nab Murphy. The ball caromed off Murphy as he reached third base and rolled into foul ground. Murphy scooted home with the go-ahead run on the miscue.

“I have been playing pretty aggressive trying to backdoor guys like that,” Desmond said. “I think that one just didn’t work out. If it doesn’t hit his body, [the] ball just sits there and nothing happens. I think given the opportunity again, I’d probably try it again. If there’s a chance to get an out there, I want to help the kid out.”

Josh Satin followed with a single on Jordan’s 84th pitch, and Johnson came to the mound to take the ball. A sacrifice fly finalized Jordan’s linescore. The Mets tacked on another pair of runs against Craig Stammen in the sixth inning as the Nationals continued to flail. There would be no comeback this time, just another possible winning streak extinguished.

“It just kind of shows you that each game is completely individual and it doesn’t really matter,” Zimmerman said. “I think last night’s win was great, but that doesn’t really get us anything today.”