Somerset 146 and 204 for 6 (Vaughan 68, Wells 2-23) need 189 runs to beat Lancashire 140 and 398 (Wells 130, Bohannon 60, Leach 3-57)
Needing 393 to beat Lancashire and keep their hopes of winning their first Vitality County Championship title alive, Somerset ended the third day's play at Emirates Old Trafford on 204 for 6, still 189 runs short of their target.
Should they succeed in their now deeply improbable task, Somerset will take their battle with Surrey into the final week of the season. However, defeat would relegate Lancashire, who will also be playing Division Two cricket next season regardless of their own results if Nottinghamshire take ten points from their final match at home to Warwickshire, who would also need five points from that game.
From yesterday's play, Luke Wells made 130 in Lancashire's second-innings total of 398 but Archie Vaughan responded with 68 to give his side some hope of mounting what would be the highest fourth-innings run-chase against Lancashire.
Those hopes were greatly diminished when George Balderson removed both Vaughan and Tom Kohler-Cadmore after tea but Kacey Aldridge and James Rew looked set to ensure Somerset suffered no further losses. Alas, Aldridge was caught by Matty Hurst off Luke Wells for 19 when there were only four balls left in the day's play.
In a slightly extended morning session, Lancashire added exactly 100 runs for the loss of their last three wickets. Wells and George Balderson went about their business carefully in the first hour, adding just 36 runs to their side's overnight score of 298 for 7.
However, a thick-edged three to third man off Aldridge took Wells to his second hundred of the season. He had faced 196 balls, hit 12 fours and a six and had batted with a fluency that made one wonder why he had struggled for so much of the season.
Wells and Balderson had taken their partnership to 135, thereby setting a new eighth-wicket partnership for Lancashire against Somerset, before Wells was leg before wicket for 130 when attempting to reverse-sweep Jack Leach.
Tom Bailey was also leg before to Leach for a single and the innings ended when Balderson was bowled by Brett Randell for 47. Leach finished with 3 for 57 and Randell took 3 for 71.
Somerset's pursuit of 393 began atrociously when Andy Umeed charged down the wicket to Bailey's second ball, tried to drive it through the covers and had his off stump knocked out of the ground.
But that misjudgement was followed by a fine 105-run partnership for the second wicket between Vaughan and Tom Lammonby, a stand that was only ended when Lammonby played over the top of a full-length ball from Wells and was bowled for 49.
By then, though, Vaughan had reached his maiden first-class fifty off 84 balls but the afternoon session still ended badly for Somerset when Tom Abell was leg before wicket to Anderson Phillip for five.
Nine overs into the evening session, Vaughan was caught at slip by George Bell for 68 to leave his side on 146 for 4 and the same combination accounted for Tom Kohler-Cadmore, who made 23 before edging a lifter to slip.
However, in the first gloomy conditions of the game, Aldridge and Rew successfully defied Lancashire's attack until Wells struck that final crucial blow. Balderson has so far taken 2 for 33 from 14 overs.
