The Nationals, meanwhile, are just a half-game behind the Phillies.
"It's a very big series," Phillies manager Charlie Manuel said. "We want to show people that we intend to stay in this thing all the way until the end. I think we're ready for it."
Still, a lot has changed since the Phillies and Nationals last met. Washington was in first place when they came to Citizens Bank Park July 8-10, but they lost two of three games. The Phillies used it as a springboard to their current success, as they are 18-11 in the second half.
Also, the Nationals' skid out of first place might have coincided with that fateful series. When they won the first game 8-7 on July 8, it was their 24th one-run victory, a major league best. However, they lost the next two games by one run, and it triggered a startling reversal of fortune -- they have lost 12 of their 13 one-run decisions since the All-Star break.
Manuel said he noted a difference in his team in the second half, and he believes taking two of three games from the Nationals might be a big reason why.
"It might have carried over into the second half," Manuel said. "Those games were real tight, and we found a way to win two of them."