New York had 30 hits during the past two games.
Mets starter Jonathon Niese never had a 1-2-3 inning, but the Nationals managed only single runs in the second and fourth against him. He allowed two runs and 10 hits in 5 2-3 innings before leaving with a left leg cramp.
"It's that kind of year," Niese said of New York's string of injuries, including first baseman Ike Davis on Saturday.
"Today was tough, it was so humid. I was crushing Gatorades and water, and still cramping up."
Scott Atchison replaced Niese with two outs in the sixth and stranded two inherited runners.
Ramos' RBI single against Atchison in the seventh made it 5-3.
"We finally got some hits with runners out there," Johnson said. "It took a while."
Murphy followed Eric Young's leadoff double with a two-base hit for New York's first run and scored one out later on Duda's single.
Another RBI single from Duda, who started for Davis, made it 5-2 in the seventh.
New York went ahead 3-0 as de Dekker's first major league home run landed in the second deck of the right-center field stands.
Ohlendorf pitched five innings, allowing four runs and nine hits. He had a 2.82 ERA in four previous starts this season.
Notes: Zimmerman had three hits. ...The Mets, with their rotation depleted by injuries, have signed RHP Aaron Harang to a minor league contract and assigned him to Triple-A Las Vegas of the Pacific Coast League. Harang was 5-11 with a 5.76 ERA in 22 starts this year for the Seattle Mariners. Overall, he has pitched 12 years in the majors, going 110-115 with a 4.29 ERA. ... The Mets also called up three players, including RHP Vic Black, who was acquired on Aug. 27 from the Pirates in a deal that sent OF Marlon Byrd and C John Buck to Pittsburgh. ... Washington also recalled RHP Ian Krol from Triple-A Syracuse and catcher Sandy Leon from Double-A Harrisburg. ... RHP Daisuke Matsuzaka (0-2, 8.68 ERA) opposes Paul Maholm (9-10, 4.37) as New York opens a three-game series at Atlanta on Monday. ... Washington plays at Philadelphia on Monday as RHP Stephen Strasburg (6-9, 2.96 ERA) faces LHP Cole Hamels (6-13, 3.58 ERA).