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
1. US Portsmouth (maroon/blue) v Horndean
US Portsmouth (maroon/blue) v Horndean. Picture: Keith Woodland Photo: Keith Woodland
2. US Portsmouth (maroon/blue) v Horndean
US Portsmouth (maroon/blue) v Horndean. Picture: Keith Woodland Photo: -
3. US Portsmouth (maroon/blue) v Horndean
US Portsmouth (maroon/blue) v Horndean. Picture: Keith Woodland Photo: -
4. US Portsmouth goalkeeper Owen Craig punches clear
US Portsmouth goalkeeper Owen Craig punches clear. Picture: Keith Woodland Photo: Keith Woodland
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