After bowling Glamorgan out for 390 in their first innings, Kent made a decent start to their reply as they reached 137 for two with Sam Northeast making 62.
However, the dismissal of Northeast, trapped lbw by Croft, was the start of a remarkable passage of play that saw Kent lose their final eight wickets for just 33 runs to be bowled out for 170.
Croft, playing in his final first-class match before retirement, finished with figures of five for 31 to help Glamorgan claim a first-innings lead of 220 and leave Kent needing to produce something special to keep alive their promotion chances.
Worse was to follow for Kent, though, as, after being asked to follow on, they lost captain Rob Key and Daniel Bell-Drummond for ducks before closing on 33 for two. Earlier, Kent had been frustrated by Glamorgan's ninth-wicket pair Jim Allenby (125 not out) and John Glover (55), who extended their partnership to 128.
Kent's problems mean that top two Yorkshire and Derbyshire are now big favourites to win promotion.
Yorkshire are 44 for three in their second innings against Essex at Chelmsford, a lead of 179 on a day when 15 wickets fell. After securing a third batting point before they were bowled out for 312 in their first innings, Yorkshire then claimed three bowling points as they dismissed Essex for 177 to earn a lead of 135.
Essex all-rounder Graham Napier took all three Yorkshire wickets in their second innings to go with his five for 65 first time around.
At Derby, hosts Derbyshire were 167 for five in reply to Hampshire's 272 when rain prevented any play after lunch, with Usman Khawaja's unbeaten 71 and nightwatchman Tony Palladino's 58 helping the home side add 117 for the loss of two wickets in the morning session
The other Division Two game saw teenage batsman Shiv Thakor (68) record his fourth half-century in nine innings to steer Leicestershire to 220 for five, 14 runs behind Gloucestershire's first-innings 234, before rain also ended play early.
Somerset put themselves on course to be Division One runners-up for only the third time in their history thanks to centuries from Marcus Trescothick and Nick Compton against Worcestershire at Taunton.
Trescothick made 146 following an opening stand of 153 with Arul Suppiah (75) while England hopeful Nick Compton, the leading run-maker in first-class cricket this summer, was unbeaten on 114 as Somerset closed on 451 for seven, a lead of 239.
In a low-scoring match at Trent Bridge, Nottinghamshire will resume their second innings on 49 without loss against champions Warwickshire, having wiped out their first innings deficit. The Bears were bowled out for 204 on a day shortened by rain in reply to Nottinghamshire's 155.
An unbeaten half-century by Luke Wells kept Sussex afloat as they reached 112 for five, a lead of 96, in their second innings against Durham at Chester-le-Street.
Meanwhile, at Liverpool, Lancashire and Surrey were left frustrated as their match fell victim to bad weather for the second day running.