Damion Easley also connected for the NL East-leading Mets, who moved five games ahead of the Phillies.
Maine (9-4) allowed two runs -- one earned -- and four hits in eight-plus innings to improve to 4-0 with a 2.13 ERA in six career starts against Philadelphia. He struck out six and had no walks.
The Phillies entered the day leading the NL in runs scored and also led the league last year. But they can't figure out Maine.
"I don't know the answer to that," Maine said when asked about his mastery of the Phillies. "I just have to go out there and pitch."
Billy Wagner relieved Maine after Jimmy Rollins led off the ninth with a double. He allowed an RBI single to Chase Utley, and walked Ryan Howard. With two on and two outs, he faced Pat Burrell as the tying run and struck him out swinging.
The last time Wagner faced Burrell, the slugger hit a game-tying home run in the ninth inning. Philadelphia went on to win that game 6-3 in 10 innings to finish off a three-game sweep at Shea Stadium earlier this month.
The injury-riddled Phillies are using three rookie pitchers in this pivotal four-game series. They were counting on a strong outing from ace Cole Hamels against Maine, but he struggled in his third bid to win his 10th game.
Hamels (9-4) gave up three runs and five hits in five innings, tying a career-high with five walks.
"At times, he was overthrowing," manager Charlie Manuel said.
Home plate umpire Ron Kulpa issued a warning to both teams after Hamels' pitch nearly hit Reyes.
Beltran homered leading off the third to make it 2-0. After Utley's sacrifice fly in the fourth cut it to 2-1, Beltran connected again for a 3-1 lead in the fifth.
Easley hit a two-run shot to straightaway center off Geoff Geary in the seventh.
Beltran was robbed of extra bases in the sixth when right fielder Michael Bourn made an outstanding running catch on his hard liner. Bourn also robbed Maine with an excellent play earlier.
"We feel good about ourselves," Beltran said. "We know the Phillies are contenders. This is a real important series."
The Phillies have lost Jon Lieber (foot) and two-time All-Star Freddy Garcia (shoulder), both likely for the season, and closer Tom Gordon (shoulder) and starter-turned-closer Brett Myers (shoulder) remain sidelined.
- The Phillies (41-39) are 0-4 when they have a chance to move a season-best five games above .500.
- Manuel, a consistent target for criticism in Philadelphia, drew cheers when he asked for the check on Hernandez's cap.
- Durbin's only other start was for Minnesota against Cleveland on Sept. 23, 2004. He lasted three innings, allowing five runs.
- Coste's pinch-hit homer was the first of his career and the fourth for the Phillies this season.
- Beltran has 19 career multihomer games, including two this season.