The Phillies (81-73) have won nine of 11 and took over the NL wild-card lead with Los Angeles' 9-3 loss to Arizona.
Howard, who leads the majors with 58 homers, was 1-for-2. He singled in the third, was intentionally walked with two outs and a runner on first in the fourth, and struck out with runners at first and third in the sixth.
Howard was intentionally walked again in the eighth after a passed ball put runners on second and third with none out. Conine followed with a two-run double that was misplayed by right fielder Joe Borchard, giving the Phillies an 8-4 lead.
Lieber (9-10) allowed two runs and seven hits, striking out six for his fifth win in six decisions.
Borchard's two-run homer off Ryan Madson in the eighth pulled Florida to 6-4. But Madson got the final out of the inning and the Phillies added two insurance runs in the bottom half.
The Marlins' playoff hopes are fading following their eighth loss in 11 games. Florida, which became the first team in history to achieve a winning record after being 20 games below .500, fell to 76-79.
"They got the big hits when they needed to and Lieber shut us down," Marlins manager Joe Girardi said.
The Phillies scored three runs in the first inning against Brian Moehler (7-10).
Rollins and Victorino hit consecutive singles. Utley followed with an RBI single to make it 1-0. Howard was hit by a pitch, drawing a warning for both teams from plate umpire Bob Davidson because Florida's Miguel Cabrera was plunked in the top of the first.
Ruiz made it 4-0 with an RBI double off the wall in left in the third. Utley and Howard hit consecutive singles to start the inning. Moehler again retired Dellucci and Burrell, but Ruiz came through with another key hit.
"He came up with big hits," Phillies manager Charlie Manuel said about the rookie. "He's getting confidence in himself."
Alfredo Amezaga drove in Florida's first two runs with a groundout in the fourth and a two-out single in the sixth.
Rollins homered to right-center in the sixth and pinch-hitter Conine added an RBI single in the inning to make it 6-2.
"That's how you play winning baseball, everybody contributing," Rollins said.
Moehler allowed four runs and eight hits in 3 1/3 innings.
Philadelphia is 35-19 since July 29, the day before it traded outfielder Bobby Abreu to the New York Yankees as part of a midseason roster purge.
- Rollins has a career-best 74 RBI.
- Ramirez, a rookie shortstop, made his 25th error in the third inning.
- The Phillies are a season-best eight games over .500 and 40-39 at home.
- Victorino and Utley each tied a career high with four hits.