It is the longest skid for the Nationals since they moved to Washington before the 2005 season, and the franchise's worst since the Montreal Expos dropped 11 games straight from June 23-July 4, 1991.
With 36 games remaining, Washington will need a decisive turnaround to avoid another unwanted milestone. The Nationals must finish 19-17 to dodge the club's first 100-loss season since the Expos went 55-107 in 1976.
Werth drove a 1-2 pitch from Steven Shell (0-1) to the seats in left-center for his career-best 17th homer.
The Phillies, who entered 1½ games behind the NL East-leading New York Mets, have won three of four after 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.
Ryan Madson (3-1) pitched a perfect eighth and Brad Lidge finished for his 31st save in as many chances. Lidge has 34 consecutive saves, dating to last season with Houston.
The Phillies, who usually have trouble with situational hitting, had three sacrifice flies. Shane Victorino doubled, tripled and scored twice.
Washington's Ronnie Belliard was 4-for-4 and Willie Harris hit a solo homer.
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 Geoff 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.
Singles by Lastings Milledge, Belliard and Jesus Flores to start the fourth put the Nationals ahead 2-1.
Ryan 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.