The Phillies' magic number for winning their second straight division championship was reduced to one after the New York Mets lost 6-1 to the Florida Marlins.
"I like our position," manager Charlie Manuel said. "I'll like it better when we're up two with one to play. That's when I'll drink champagne, dance and sing."
Fans kept a close eye on the out-of-town scoreboard, cheering when the Marlins took a 2-0 lead in the first inning at Shea Stadium.
A few minutes later, Howard got them roaring when he connected to give the Phillies a 3-0 lead in the first. The sellout crowd screamed "MVP! MVP!" and waved their white-and-red "Fightin' Phils" towels in appreciation.
They went wild in the ninth after Manuel got into a heated argument with plate umpire Andy Fletcher and was ejected for arguing that Kory Casto had struck out. The Philly faithful gave Manuel a standing ovation and chanted his name.
"We knew what needed to be done and took care of our own business," Howard said. "We have everything in our own hands right now."
Chase Utley had a three-run double and Howard drove in four runs as Philadelphia built a 7-1 lead off Collin Balester (3-7).
The Nationals chipped away against Blanton (4-0), but four relievers combined to toss three scoreless innings.
The Phillies reached 90 wins for the 12th time in the franchise's 126-year history. They hadn't done it since winning 97 games in 1993.
Washington lost its 100th game, the first time the franchise has lost that many games since the Montreal Expos were 55-107 in 1976.
"A loss is a loss to me," manager Manny Acta said. "Once you lose over 90, it's one too many."
Blanton allowed four runs - one earned - and five hits. The right-hander was 5-12 with Oakland before joining the Phillies in a midseason trade.
Jayson Werth and Utley hit one-out singles in the first to bring up Howard. The 2006 NL MVP drove an 0-1 pitch into Philadelphia's bullpen in right-center.
Howard is batting .349 (29-for-83) with 11 homers and 32 of his major league-best 146 RBIs this month. He has raised his overall average to .250 and is making a strong push to win his second MVP award in three years, though teammate Brad Lidge deserves serious consideration.
Lidge, who is 40-for-40 in save opportunities, was chosen the Phillies' most valuable player by the local chapter of the Baseball Writers Association of America.
"His numbers prove he's very deserving," Manuel said when asked about Howard's chances. "His production speaks for itself."
The Phillies chased Balester with four, two-out runs in the second inning to take a 7-1 lead.
Greg Dobbs led off with a double. Jimmy Rollins walked with two outs and Werth reached on an infield single to load the bases. Utley missed a slam by a few feet, clearing the bases with a double off the left-field wall. Howard followed with an RBI double to left.
The Nationals scored a pair of unearned runs in the fifth after Howard booted Pete Orr's grounder with one out. Emilio Bonifacio had a sacrifice fly and Cristian Guzman singled in a run to cut it to 7-3.
Casto hit a solo homer in the sixth to get the Nationals within 7-4. Umpires used instant replay to make sure the ball cleared the right-field wall before it fell onto the field after a fan dropped it.
Balester allowed seven runs and seven hits in 1 2-3 innings.
Notes: Rollins set a career-high with his 47th stolen base. He's been caught just three times. ... Howard's 32 RBIs this month are a team record for September. Gary Matthews, now a broadcaster with the Phillies, had 31 RBIs in September 1981. ... Nationals 3B Ryan Zimmerman (flu) sat out his second consecutive game. ... Howard leads all first basemen with 19 errors. ... The Phillies are 9-4 in Blanton's 13 starts