Greg Halford's header midway through the second half rescued a 1-1 draw for Portsmouth and checked Preston's hopes of a great escape.
North End led in the 32nd minute as in-form striker Iain Hume grabbed his sixth goal in as many games.
Preston proceeded to dominate the periods either side of half-time and a fourth win in five games looked on the cards for Phil Brown's men.
However slack marking allowed Halford to nod an equaliser and some shaky defending in the closing stages meant Portsmouth might have claimed all three points.
Preston sit ahead of bottom club Sheffield United on goal difference and are seven points from npower Championship safety with six games to play.