Leading off the top of the third inning, Cubs catcher Koyie Hill hit a routine ground ball to third baseman Ryan Zimmerman, who threw away the ball for a two-base error.
It was the 13th error of the season for Zimmerman, who has had problems throwing to first base for most of the year.
"I realize it's a part of my game that I need to improve on," said Zimmerman, who added that he had a problem gripping the ball on the Hill play. "It happens sometimes. It's something I need to work on. [The errors] can't happen."
Riggleman expressed his own theory as to why Zimmerman had a problem throwing the ball to first.
"Ryan gets in between whether he is going to pick it up and fire or take another step and throw it on the run. I think we saw a little in between there, and it didn't come out of his hand very well," Riggleman said. "We are going to put the work in to cut down on that."
Chicago took advantage of the error. Two batters later, Ryan Theriot doubled near the left-field line to drive in Hill and give Chicago a 1-0 lead.
Lannan made another quality start for Washington, giving up three runs -- two of which were earned -- in 6 2/3 innings. But Cubs right-hander Rich Harden was better, lasting six innings without giving up an earned run.
Before the game, the Nationals watched video of Harden in which he was throwing in the low 90s. But to their surprise, on this night Harden was throwing gas, with a fastball clocked as high as 98 mph. It's obvious that the All-Star break did him some good.
"He was real good," Riggleman said. "I was in the American League for one year, and I heard a lot about Rich Harden [who was with Oakland]. I never saw him. I heard a lot of good things. He got traded to the Cubs, and I could see what the people were excited about.
"We were kind of hoping to stay close and eventually get his pitch count up to where he was out of the game and we would do something against their bullpen. But the bullpen guys came in and did a good job, too."
The Cubs gave Harden additional run support in the sixth inning, when Derrek Lee hit his 18th home run of the season.
Washington scored its first run of the game off Harden in the bottom of the sixth inning, when Nyjer Morgan scored on a Nick Johnson groundout.
The team had had two other chances to score more runs but couldn't get the job done.
In the third inning, Alberto Gonzalez hit a one-out triple but was left stranded when Lannan struck out and Morgan grounded out to end the frame.
Two innings later, the Nationals had runners on first and second, two outs and Lannan at the plate. A .125 hitter this season, Lannan grounded into a force play on the first pitch.
"It's always frustrating [when you can't score runs]," Zimmerman said. "I think Harden had his good stuff. He was throwing his pitch at any time. The way he was throwing tonight, it's tough to get hits and get on base at all."
"Our starting pitching has been great," Harden said. "I think it's the key to the game, and it's tough if you're hot and cold and hot and cold. Something I want to do in the second half is be consistent with my starts. Tonight I really felt great with some of the stuff I've been working on. I'm feeling the difference, and now I have to go out in my next start and do it."
The Cubs had scored four more runs by the time the Nationals came up to bat in the bottom of the ninth inning, when Zimmerman hit a solo homer over the right-field wall off reliever Kevin Gregg.
After the game, Riggleman tried to put a positive spin on his team, which he believes has the talent to win games.
"Maybe I'm just excited about managing, but I feel good about the ballclub," Riggleman said. "With our record, that's not what some fans want to hear, but I feel we are going to turn this thing around. Those guys in the room are going to be the reason. I know they care. They are going to get it done."