Zack Willett opened the scoring with his 35th league and cup goal of the campaign.
Midfielder Tommy Tierney wrapped up the win by scoring twice, with all the goals coming in the first half.
Horndean still have two games in hand on Portchester and travel to the On-Site Group Stadium next month.
Pictures by Keith Woodland
17. Horndean's Rudi Blankson, right.
Horndean's Rudi Blankson, right. Picture: Keith Woodland Photo: Keith Woodland
18. Tommy Tierney about to score his second goal and Horndean's third.
Tommy Tierney about to score his second goal and Horndean's third. Picture: Keith Woodland Photo: Keith Woodland
19. US Portsmouth (maroon/blue) v Horndean
US Portsmouth (maroon/blue) v Horndean. Picture: Keith Woodland Photo: Keith Woodland
20. Horndean captain Sam Hookey (white)
Horndean captain Sam Hookey (white). Picture: Keith Woodland Photo: Keith Woodland
5 / 7