Halladay (7-3) allowed 10 hits and four runs over 7 innings, striking out five without a walk. Howard and Ibanez hit back-to-back homers in the fourth and each drove in a run in Philadelphia's seventh-inning rally for a 5-4 win Monday.
Halladay gave up solo home runs to Michael Morse, Danny Espinosa and Laynce Nix, the first time this season he's given up multiple homers in a game. Halladay had allowed a total of two home runs in his first 11 starts.
Ryan Madson pitched a perfect ninth for his 12th save in 12 chances.
Halladay is 11-1 against the Nationals and Expos franchise since losing June 28, 2002, in Montreal.