WASHINGTON—Jeff Baker’s incredibly wayward throw gave the Washington Nationals a pair of runs in the first inning Tuesday night, and Ross Detwiler won his season debut in a 3-2 victory over the Cubs.Baker was only 30 feet or so from second when he fielded a routine grounder with the bases loaded and none out, but his throw sailed more than 10 feet wide of the bag and into left field. Instead of allowing one run on a double play, the Cubs were in a 2-0 hole that became 3-0 on Wilson Ramos’ infield single.That was just enough of a cushion for a Nationals cadre of pitchers that included Detwiler (1-0), recalled from Triple-A before the game because Washington needed an extra starter due to a recent doubleheader. Detwiler took a shutout into the sixth but was removed after giving up Aramis Ramirez’s two-run homer. Detwiler allowed four hits over 5 1/3 innings with one strikeout and no walks. The Cubs were shut out the rest of the way by Todd Coffey, Sean Burnett, Tyler Clippard and Drew Storen (21st save) as the Nationals moved back over the .500 mark thanks to their sudden knack for winning one-run games. Each of Washington’s last nine wins has come by one run or in extra innings, and the club is 12-3 in one-run games since June 1. The Nationals also improved to 4-5 under new manager Davey Johnson, and they continue to get hot-and-cold performances from high-priced slugger Jayson Werth. Werth finally drew some cheers with a daring steal of third before scoring on a wild pitch to win Monday’s game in the 10th inning, but on Tuesday he heard boos again when he struck out twice — including once on a meager check swing — and grounded into a double play with two on in the fifth inning. The Nationals left 10 runners on base — six by Werth. The Cub pitcher was also making his season debut. Ramon Ortiz (0-1) had the rough first inning — exacerbated by Baker’s miscue — but he settled down and didn’t allow another run. He went six innings, gave up seven hits and struck out seven in his first major league start since May 19, 2010, for the Los Angeles Dodgers. The first three batters to face Ortiz singled to right field, but he induced a grounder by Laynce Nix that appeared set to get him two outs — until Baker’s did-that-just-happen E-4. Baker, in the starting lineup to give All-Star Starlin Castro a day off, left the game in the bottom of the third with lower back spasms.Ortiz’s contract was purchased from Triple-A before the game, the latest move in a season-long pitching carousel for the Cubs. Ortiz became the 10th Cub starter this season, his services needed because Carlos Zambrano is on the disabled list with a sore back. Four of Chicago’s five starters from the opening day rotation have been on the DL this season.

NOTES—Ortiz became the Cubs’ second fill-in starter in two days. Casey Coleman started Monday because Ryan Dempster was recovering from a stiff lower back. …The Cubs also placed reliever Marcos Mateo on the 15-day disabled list with a sore elbow. Mateo was flown to Chicago and had an MRI on Tuesday, but the team doesn’t expect to have the results for another day or two. … The Cubs flip-flopped a move they made Monday, recalling reliever Chris Carpenter from Iowa and optioning Coleman to the Triple-A club. … First lady Michelle Obama was cheered as she took part in a pregame ceremony honoring children of military families. She watched from the infield as a boy from a military family threw the ceremonial first pitch to Nationals 2B Danny Espinosa. Mrs. Obama then gave Espinosa a hug. … The security measures for the first lady caused Johnson to arrive later than he planned at the ballpark. Johnson wanted to get to his office early for some “homework time” as he learns more about his roster. He estimates that he’s “about 80 percent there” learning the players’ various statistical tendencies. ..