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
25. US Portsmouth's Lamin Jatta, right.
US Portsmouth's Lamin Jatta, right. Picture: Keith Woodland Photo: Keith Woodland
26. Zack Willett has just headed Horndean in front.
Zack Willett has just headed Horndean in front. Picture: Keith Woodland Photo: Keith Woodland
27. Horndean's Fuzz Kanjanda, right
Horndean's Fuzz Kanjanda, right. Picture: Keith Woodland Photo: Keith Woodland
28. US Portsmouth's Simon Woods on the ball
US Portsmouth's Simon Woods on the ball. Picture: Keith Woodland Photo: Keith Woodland
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