With the rain coming down steadily and no end in sight, John Maine knew he had to be efficient. It’s a good thing he was.

The Mets starter made quick work of Washington, allowing only a first-inning single in five innings, and New York salvaged a four-game split with a 5-0 victory over the Nationals yesterday that was shortened to 4 1/2 innings by rain.

David Wright drove in two runs and Ramon Castro made sure the Mets didn’t miss All-Star catcher Paul Lo Duca, who strained his right hamstring Saturday night and was unavailable. The backup catcher went 2-for-2 with a home run, a double and two RBIs.

New York remained undefeated in seven four-game series this year. The last time the Mets lost a four-game series was September 22-25, at home against the Nationals.

The rain started before the game and never let up. Despite occasional downpours, intermittent thunder, and even a few flashes of lightning, officials seemed hesitant to call the game, perhaps because this was the Nationals’ last visit to Shea before the final week of the season.

“It was a steady rain right from the beginning,” Nationals outfielder Ryan Church said. “It wasn’t like it was misting or drizzling. It got locked in over us and it was coming down steadily.”

Less than 10 minutes after the game became official — with one out in the bottom of the fifth — plate umpire Mike Winters went out to the mound and called out the grounds crew to cover the infield. The game was called after a wait of 91 minutes.

Maine cruised, retiring his final 14 batters after a one-out single by Ronnie Belliard in the first. He threw 62 pitches, 44 for strikes. He got two foul popouts to start the fifth and struck out Nook Logan swinging to end it.

It was Maine’s first one-hitter and his second career complete game. His first one was a four-hitter on July 21, 2006, against Houston.

“He was very efficient, kept going after guys,” manager Willie Randolph said. “He’s been our stopper and that’s what stoppers do.”

Maine (12-5) struck out five without a walk and won for the sixth time in his last eight starts. His control has been a big factor; he’s walked a total of three in those six wins, with 37 strikeouts. By the fifth inning, Maine knew the game was in jeopardy of being called at any moment. Still, he didn’t change his strategy to try to get the game to be official.

“By the fifth inning it was bad,” he said. “But you go out there and do your job until they tell you it’s over.”

Castro hit a fastball an estimated 425 feet to center for a two-run homer in the fourth to give the Mets a 4-0 lead.

Castro has hit safely in his last 11 starts, and is 17-for-38 in those games. Despite just 101 at-bats, he is tied for fourth on the team with seven homers.

“They don’t give me the chance I want, but I know what I have to do,” Castro said. “I feel I’m ready.” The Mets wasted no time against left-hander Billy Traber, who found himself trailing six pitches in. Jose Reyes lined a leadoff double, and Lastings Milledge followed with a single up the middle to score Reyes.

Reyes struck again in the third, doubling and swiping his major league-leading 49th base. Wright singled Reyes home to make it 2-0.

Traber (2-2) was ineffective in his second start of the year, allowing five runs and eight hits in 3 2-3 innings. He left after walking Milledge, and Wright greeted reliever Chris Schroder with an RBI single, scoring Ruben Gotay for the Mets’ final run.

Traber was moved into the starting rotation on July 20 to replace the injured Jason Simontacchi. Traber had pitched 24 games out of the bullpen before that.

“I think he gave us what he had,” manager Manny Acta said. “He’s a relief pitcher but we needed him today and he did as well as he could.”

The Nationals failed to clinch their first winning month of the season. They are 13-12 with one July game remaining, at home Tuesday against Cincinnati.

Notes: C Lo Duca’s MRI exam yesterday showed a strained hamstring, and he hopes to be ready to catch Tuesday when Tom Glavine goes for his 300th career win at Milwaukee. Lo Duca said he tore scar tissue and that he wasn’t concerned. When told of Lo Duca’s optimism, Randolph said: “If he is, then I am.” … CF Carlos Beltran’s MRI showed a strained oblique. Randolph said Beltran would be re-evaluated in a couple days. … Milledge extended his hitting streak to nine games and is batting (.424) 14-for-33 with seven RBIs during that span. … The win was the third five-inning win in Mets history, according to the Elias Sports Bureau. The other two were both at Shea Stadium against the Montreal Expos in 1971.