Somerset 167 (Harmer 5-64) and 172 for 4 (Lammonby 59, Abell 54*) need another 294 runs to beat Essex 462 for 9 dec (Critchley 121, Khushi 56*) and 170 for 7 (Critchley 52, Aldridge 4-36)
Matt Critchley scored his seventh 50-plus score of the season before taking two wickets as Essex pushed for a crushing victory over Somerset in the LV=Insurance County Championship.
All-rounder Critchley notched up 52, to go with his first-innings century, as Essex set Somerset an unlikely 466 runs to win in just under five sessions.
Somerset's batters, headed by Tom Lammonby and Tom Abell's half-centuries, took advantage of a heavy-rolled pitch to bed in, show fine grit, and bolster their chances of saving a draw.
Critchley struck with his first ball to pin Lammonby for a two-hour 59 and also dismissed George Bartlett as Essex need six wickets on the final day to win at the Cloud County Ground, Chelmsford for the first time this season - Somerset still need 294 runs.
Essex's brain trust had decided not to enforce the follow-on - with the decision hinging on giving the bowlers a longer break, the offset being less time to bowl Somerset out.
They had taken their 295 first-innings lead to 310, but lost Nick Browne in seven overs on the second evening.
But the third morning was a meandering session where Essex seemed in no rush to declare, albeit against some well-directed bowling. A total of 113 runs were scored in the session with four wickets falling.
New Zealand fast bowler Matt Henry pinned Division One's leading run scorer leg before in the sixth over of the day.
Paul Walter showed a few moments of intent in his 25 before a drive with little footwork was well caught at gully, before Feroze Khushi fended a ball which stopped in the pitch to first slip.
Critchley had shown impetus, as he had during his 121 in the first innings, to push the lead to 450 - the loose plan to bring about a declaration.
The former Derbyshire player carted Aldridge over midwicket for six before clobbering debutant spinner Shoaib Bashir back over his head.
His half-century came in 64 balls and was his sixth of the season. No one in the country has passed fifty more than Critchley's six in the Championship - although he has only converted to three figures once.
After Simon Harmer had picked out long on in a post-lunch frenzy, Critchley departed for 52 after lobbing a reverse pull shot softly to short fine leg.
Will Buttleman and Ben Allison both stuck sixes as the lead soured past 450, before Tom Westley pulled out with 55 overs left in the day, and 151 in the match.
Lammonby and Sean Dickson - who had carried his bat in the first innings - looked in little danger before the latter surprisingly hoicked Jamie Porter to midwicket.
Essex tend to pin their hopes on Harmer in these situations, especially after his first-innings five-for. He was introduced in the seventh over.
The South African bowled 14 fruitless overs before Critchley was thrown the ball to make something happen. He did immediately as a drag down stayed low and hit Lammonby - who had struck his third fifty of the season - on the pads.
After two overs from the River End, the leg spinner switched to the Hayes Close End and two balls later influenced an edge from Bartlett which was well caught at first slip.
Abell's fifty came in 124 balls but Tom Kohler-Cadmore was lbw to Harmer to the last ball of the day having put on 55 with his captain.
