Somerset took charge against Hampshire as Marcus Trescothick and Justin Langer piled on the runs at Taunton.
Trescothick (151) and Langer (188) shared a stand of 272, Somerset's best second-wicket partnership for 84 years.
A series of thumping drives carried Trescothick to his 43rd first-class ton while Langer punished anything loose.
An unbeaten 75 by Zander de Bruyn further compounded Hampshire's woes as the hosts ended day three 326 runs ahead with five wickets remaining.
Langer, resuming on 34, could have gone early on but Greg Lamb's throw from square leg narrowly missed the stumps with the Australian well out of his ground.
The error was the only aberration in the partnership as Trescothick, displaying the qualities desperately lacking at the top of England's order, pulled a six off Sean Ervine into the netting guarding the construction work on one side of the ground.
The former England opener was finally caught down the leg side by substitute wicket-keeper Tom Burrows, while Langer eventually edged a Lamb off-break behind.
Hampshire's relief did not last for long as De Bruyn, despite a slow start, piled on more runs with an 89-ball half-century.
~RS~q~RS~~RS~z~RS~06~RS~)