Campbell directed a Matt Taylor corner past Paddy Kenny for Pompey's second goal after Rob Kozluk's bizarre own goal had cancelled out Rob Hulse's opener for the away side.
Noe Pamarot added a third for his second in as many weeks as Neil Warnock's side fell to their first defeat in five games.
Following their 2-2 draw at Arsenal last weekend Harry Redknapp made just one change with midfielder David Thompson sacrificed in place of Benjani who recovered from his hamstring injury sustained against Everton a fortnight ago.
Neil Warnock was boosted by Danny Webber's presence after the former Manchester United trainee overcame a midweek virus.
Despite Pompey looking to reclaim a top-four position they have occupied for most of the season, they fell behind to an early blow when full-back Derek Geary was given time to turn back on his right foot and send a curling cross to the far post.
Campbell's poorly timed leap allowed Hulse to gamble and steal in behind the former England international to thump a header into the far corner past David James.
The homes side tried to hit back with half chances falling to Kanu and Benjani, but the passing game that had seen them rise up the Premiership table was largely absent from the opening 20 minutes as the Blades midfield dominated Sean Davis and Pedro Mendes.
It was Davis who was nearly culpable for a second Sheffield goal when a hopeful punt upfield saw Webber bearing down on the shaggy-haired midfielder.
Davis attempted to clear the ball over his head but missed completely leaving Webber one on one with James, only to slash his effort wide of the far post.
The shock of nearly going two behind appeared to wake a dormant Pompey side and Kanu and Benjani combined well on the edge of the area only for the Nigerian to overcook his strike and see it soar into the stands.
Man of the moment Taylor then went looking for Kanu but his drilled cross on 32 minutes was well intercepted by Kenny.
Gary O'Neil's attempt to bend around Kenny had the Sheffield United goalkeeper sprawling to his left to parry the ball straight to Benjani, fortunately for the Blades the Zimbabwean was offside and play was halted.
Emerging from Pompey's little spell of pressure Warnock's side then fashioned a couple of chances of their own just before half-time.
Young Stephen Quinn fired straight at James from 20 yards before Webber's twisting run in which he evaded Linvoy Primus and Mendes was only ended when the striker dragged his shot wide.
Redknapp initially decided against throwing the half-fit Andrew Cole into action and his decision proved correct when Portsmouth drew level through the heel of Phil Jagielka and the knee of Kozluk.
Glen Johnson clipped the ball down the line for Benjani to chase, as he dallied on the ball it appeared Geary would make a challenge, but he still managed to get his cross in only for it to deflect first off Jagielka and then Kozluk's despairing lunge on the line only helped the ball over the line.
Minutes later and they had another when Taylor's low inswinging corner allowed Campbell to race across Chris Morgan and beat his marker to the ball to head his first goal for the club since his summer move from North London.
Campbell's goal sprang the game into life and it began to resemble a basketball match before Pompey put the result beyond doubt, again from a corner.
This time Mendes swung the ball in and Pamarot arrived late and without company to finish for, remarkably, his second goal of the season.