Half-centuries from Gloucestershire trio Alex Gidman, Benny Howell and Chris Dent couldn't stop Northamptonshire from closing in on victory at Bristol.
The hosts, trailing by 212 after both sides had batted once in the contest, reached stumps on 280-9, meaning they lead by just 68 heading into the final day.
Andrew Hall and Steven Crook both claimed three wickets while Trent Copeland picked up 2-53 to help push Northamptonshire to the brink of a second success in Division Two of this year's LV= County Championship.
Gloucestershire began the day on 34-1 and soon lost Dan Housego, Australian Copeland getting him caught behind with the fifth ball of the morning session.
Gidman and Dent applied themselves well to add 56 in 21 overs, only for the latter to perish for an even 50 when he deflected a David Willey delivery onto his stumps as he attempted to withdraw his bat.
Hamish Marshall managed just four but Gidman and Howell put on 69 for the fifth wicket with few alarms.
Former skipper Gidman even hit off-spinner James Middlebrook for a straight six as he appeared on course for a century, only to perish for 87 when he nicked a delivery from Hall to Copeland at first slip.
Although Will Gidman fell cheaply soon after, Howell and Cameron Herring (32) showed some aggression as they added 54 against the second new ball.
Howell progressed to a patient 142-ball half-century, with five fours, before he edged Hall to David Sales at second slip to give the South African his third scalp of the innings.
Herring chipped Crook to mid-wicket to depart for 32 and the same bowler got rid of David Payne for a duck, leaving Jack Taylor (30 not out) to hope for a miracle with only last man Liam Norwell for company.