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
21. Horndean's Ethan Robb is booked by referee Harry Scott
Horndean's Ethan Robb is booked by referee Harry Scott. Picture: Keith Woodland Photo: Keith Woodland
22. Referee Harry Scott about to book US Portsmouth manager Frazer Quirke
Referee Harry Scott about to book US Portsmouth manager Frazer Quirke. Picture: Keith Woodland Photo: Keith Woodland
23. Horndean's Tommy Tierney scores his side's second goal
Horndean's Tommy Tierney scores his side's second goal. Picture: Keith Woodland Photo: Keith Woodland
24. Horndean's Tommy Tierney (white)
Horndean's Tommy Tierney (white). Picture: Keith Woodland Photo: Keith Woodland
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