The damage was done before the break, with Ronan Curtis’ neat finish adding to an early Ryan Williams opener.
Some neat possession play ensured that the visitors were unable to really get into the game and although they had a few moments late on, Craig MacGillivray was rarely troubled.
Jake Beesley did score for Dale deep into stoppage-time, but it proved to be no more than a consolation.
Cowley made one enforced change from the side that had won at Shrewsbury the previous week.
With John Marquis starting a three-match ban, Curtis – back from international duty – took his place up front.
It meant that former Rochdale midfielder Andy Cannon had to be content with a place among the substitutes.
The sun was shining on the south coast and the hosts were basking underneath the blue skies as they took an early lead.
Marcus Harness got into the area and sent a low ball towards the far post, where WILLIAMS was waiting to bundle over the line.
Pompey were passing the ball patiently around the pitch and looking to find another goal, with Harness causing problems with his jinking runs.
It was Michael Jacobs who should have doubled the advantage on 19 minutes, however, scuffing his shot narrowly past the post after being played through by Curtis.
He tried again with an acrobatic effort from a Callum Johnson cross, but was unable to guide the ball on target.
Pompey did have a cushion towards the end of the half, though, as their dominance in possession paid off.
CURTIS collected a pass from Jacobs and advanced into the box before angling a precise low strike right into the bottom corner.
Gavin Bazunu was then alert to gather the ball after Harness had been found by Williams and tried to pick out Curtis.
Rochdale had barely threatened, but a long kick from their keeper sent Beesley through on goal in stoppage-time.
MacGillivray was off his line to deny the forward, however, and Sean Raggett hooked clear the loose ball.
Pompey did not abandon their passing principles once the action restarted and a nice move down the right provided another opening.
Harness swapped passes with Johnson and found Curtis, whose ball into the middle could not quite be flicked home by Williams.
Lee Brown then picked up possession and his cross-shot was gathered by Bazunu, as Rochdale held on.
The Blues were still controlling both possession and territory, with their opponents causing very few problems.
Bazunu was not having much to deal with at the other end, but the hosts’ healthy lead meant that was not causing much concern.
Cowley freshened things up on 65 minutes, with Tottenham loanee Harvey White coming on for Jacobs.
Rochdale did begin to look a bit more threatening as the contest neared its conclusion and Johnson did well to stop Kwadwo Baah getting a shot away.
The lively Harness made way for Cannon for the closing stages, with Pompey shoring up their midfield.
Charlie Daniels also came on for the tidy Ben Close, as the Blues seemed to be comfortably seeing out the final few moments.
But there were a few nerves when Beesley turned home Alex Newby’s low cross in stoppage-time before the final whistle blew and Pompey could celebrate a third straight win.
Pompey (4-2-3-1): MacGillivray; Johnson, Whatmough, Raggett, Brown; Naylor (c), Close (Daniels 87); Harness (Cannon 82), Williams, Jacobs (White 65); Curtis
Goals: Williams 5, Curtis 37
Subs not used: Ward, Bolton, Nicolaisen, Byers
Rochdale (3-5-2): Bazunu; O’Connell (c), McShane (Roberts 53), Osho; Keohane (Newby 82), Shaughnessy (Dooley 68), Rathbone, Grant (Morley 53), Done; Beesley, Baah
Goals: Beesley 90+3
Subs not used: Lynch, McNulty, Hopper
Referee: Lee Swabey