"These are the games that turn good teams into great teams," Mets third baseman David Wright said. "Great teams dig down deep in games like this."
Maine tossed seven innings of three-hit ball (all soft singles). He matched zeros with oft-injured All-Star Mark Prior, who threw 5 2/3 hitless innings before departing after 103 pitches.
"That was as far as he could go," Cubs manager Dusty Baker said. "We won the series. It's some consolation. The team with the best record in the league, we played tough."
Prior and reliever Will Ohman held the NL East-leading Mets without a hit for six innings before Valentin blooped a one-out single to center off Roberto Novoa in the seventh -- a ball that could have been caught by shortstop Ronny Cedeno.
He got back far enough but didn't appear to see it well and never had the ball lined up. It fell beyond Cedeno's outstretched glove and was immediately scored a hit.
After Wright was intentionally walked to load the bases, Valentin lined a 1-2 pitch to center, giving the Mets 15 wins in their final at-bat this year.
The 36-year-old Valentin was signed last offseason to be a role player. But he's been an extremely pleasant surprise since assuming the everyday job at second base.
"I've got something to prove here," he said, "not just to the players, but to the coaches and the manager and the front office."
Aaron Heilman (1-3) worked a scoreless inning for the win.
After allowing eight runs in each of its previous three games, New York got an outstanding performance on the mound from Maine -- again.
Mets manager Willie Randolph went to the mound during the sixth and challenged Maine to get out of a jam and finish strong. The right-hander responded.
"He really showed me a lot," Beltran said. "He's got a good fastball."
Maine matched a career high with seven strikeouts and extended his scoreless streak to 17 innings since July 8 against Florida.
Still, with Pedro Martinez ready to come off the disabled list, Randolph would not guarantee Maine a start on the upcoming road trip.
"We'll see," Randolph said. "He needs to solidify a spot. Seventeen scoreless innings for him right now, that's good."
Maine was coming off a four-hit shutout against Houston last Friday.
"It's more of a confidence thing," he said. "Before, I thought I had to make the perfect pitch in every situation."
Prior made his sixth start of the season and second since his latest stint on the disabled list -- he was sidelined with a strained muscle on his left side. He lowered his ERA from 8.14 to 6.60 but is winless in nine starts since last September, the longest drought of his career.
The 25-year-old Prior walked four in the first four innings, including the leadoff batter three times. He showed some frustration but never folded.
"It's starting to come around," Prior said. "I've had a lot of freak things happen to me. ... I think I've got a lot of great years ahead of me."
That's when he appeared to run out of gas.
Prior walked slowly back to the mound and was checked by Baker. Prior stayed in the game but never looked comfortable during a five-pitch walk to Beltran. Baker then came back out and removed the right-hander.
"I was on a short leash going into that sixth inning," said Prior, who had some cramping in a finger on his pitching hand late in his outing.
Prior threw only 56 strikes. He walked a season-high five and fanned three.
Ohman retired Delgado to end the sixth.
Mets rookie right-hander Brian Bannister, on the 60-day disabled list because of a hamstring injury, allowed four runs -- three earned -- six hits and four walks in five innings in a rehab start for St. Lucie at Lakeland in the Florida State League. Bannister, out since May, threw 83 pitches and had one strikeout. ... The Cubs plan to call up a pitcher from Triple-A Iowa to start Thursday night against St. Louis. ... Martinez is slated to start Friday night in Atlanta. Right-hander Heath Bell will be optioned to Triple-A Norfolk to make room on the roster. ... Chicago was trying for its first sweep at Shea Stadium since a four-game set in August 1991.