Werth hit a tiebreaking solo homer in the eighth inning to lift the Phillies over Washington 5-4 on Tuesday night, the Nationals' 11th straight loss.
The losing streak is the longest for the Nationals since they moved to Washington before the 2005 season. It is the franchise's longest since the Montreal Expos dropped 11 straight from June 23-July 4, 1991.
Werth drove a 1-2 pitch from Steven Shell (0-1) to the seats in left-center for his career-best 17th homer. The right-handed hitting Werth has platooned most of the season with Geoff Jenkins in right field. He's gotten more starts against right-handers lately, including this one.
"I like Werth's talent," manager Charlie Manuel said.
Despite a stiff wind blowing in, Werth knew his shot was going out once he connected.
"I hit it pretty well," he said. "It didn't go as far as I thought."
The Phillies remained 1½ games behind the NL East-leading New York Mets with their third win in four games since losing four straight in Los Angeles.
Jimmy Rollins got a mixed reaction from the crowd in his first home game since calling Philadelphia fans "frontrunners" last week in a television interview.
The reigning NL MVP heard boos and cheers each time he came to bat and went 0-for-4.
"There's been a lot worse booing than that," he said. "If they want to boo me, that's fine. Just keep it off everybody else."
The Phillies, who usually have trouble with situational hitting, had three sacrifice flies. Shane Victorino doubled, tripled and scored twice.
"Another tough loss," third baseman Ryan Zimmerman said. "We played a good game, but a couple things hurt us."
Victorino's speed helped the Phillies score the tying run in the seventh. After Pat Burrell led off with an infield single, Victorino reached on a fielder's choice. He advanced to third on Greg Dobbs' single to left-center and scored on Chris Coste's sacrifice fly to right.
Victorino started a two-run fifth with a triple to left-center. He scored when Harris dropped Dobbs' shallow fly to left. Dobbs stole second, went to third on Coste's single and scored on pinch-hitter Jenkins' sacrifice fly.
Harris gave the Nationals a 1-0 lead in the first when he drove one out to right.
Ryan Howard singled leading off the bottom of the second, advanced to third on Victorino's one-out double and scored on Dobbs' sacrifice fly to tie it.
Zimmerman had an RBI triple in the fifth and Belliard singled him in with two outs to make it 4-1.
Phillies starter Joe Blanton allowed four runs and eight hits in five innings in his sixth start since joining the Phillies in a trade with Oakland.
Nationals starter Jason Bergmann gave up four runs -- three earned -- and six hits in 6 1/3 innings.
"Bergmann threw well," manager Manny Acta said. "We're not helping ourselves when we get good pitching."
Belliard is batting .440 (22-for-50) in August. ... A crowd of 44,143 was the 40th sellout in 60 home games at Citizens Bank Park. ... Blanton is 1-0, but the Phillies are 4-2 in his starts.