Pompey thought that they had stolen a point when Gary O'Neil struck a equaliser in 78th minute to cancel out Peter Ndlovu's 24th minute goal.
However with three minutes to go United where gifted a goal scoring opportunity when Linvoy Primus failed to clear a header, in which fell to Peter Ndlovu who slipped in a pass into Michael Brown who slotted past keeper Shaka Hislop to send the Blades into a frenzy.
The Fratton Park Club are just four points above second place Leicester City but Pompey are in their bad spell and could see them in second place pretty soon. United however are now 10 points from top of league with two games in hand.
Peter Ndlovu, the Zimbabwean International should have taken the lead within 12 minutes but he failed to hit his shot cleanly. Although on 24th minute he made no mistake, Tonge wavering in and out
around the defenders and put in Ndlovu with 16 yards away from goal, his left footed shot curled round Hislop and into far bottom of the goal.
Moments later United almost took the lead a second time, Ndlovu put in a beautiful cross to Brown, who beat off his marker Primus but his shot hit Hislop's elbow and stuck the crossbar before landing on the roof of the goal.
The home side soon improved after half-time and almost scored, only the woodwork could stop them from doing so on two occasions. Cameroon man Vincent Pericard went on a solo run before shooting across Kenny's goal which struck the upright on 52 minute, Moments later a Paul Merson free-kick also hit the post.
Three minutes later there attacking skills paid off when a move involving Merson saw Todorov slip the ball through to O'Neil who chipped the ball over Kenny from close range. Though of course like in many games it was United who had the last stay when Brown struck to gave the Blades all three points.