Colby Bishop netted his 20th goal of the season as Pompey won on the road at Accrington.
Lukas Jensen was sent off for the home side after bringing down Paddy Lane and John Mousinho's men quickly broke the deadlock through a Joe Pigott header.
Reeco Hackett doubled the lead in the second half, only for Pigott's challenge for the ball with Toby Savin to also earn a red card.