It was a day to forget for the County as their bowlers toiled badly, with Kiran Carlson and Chris Cooke both hammering unbeaten centuries as the home side powered their way to 462 for four at the close.
Luke Procter claimed the only two wickets to fall in three tough sessions, and Glamorgan will go into the final day on Wednesday leading 10-man Northants by 247 runs.
The County are a man light following the ankle injury suffered by Gareth Berg in the warm-up ahead of day two, and that factor certainly didn't help the bowling attack's cause as they struggled for wickets.
Procter ended the day with three for 77 from 20 overs, but there was no joy for anybody else, and bowling coach Liddle admitted: "It’s a tough day.
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"It was always going to be made harder by Gareth Berg going down which means we’ve been a bowler light, and we would have liked Simon Kerrigan to bowl more overs in the second innings and not the first.
"We weren’t able to string too many good overs together but you’ve got to give Glamorgan’s batsmen credit.
"They were busy and kept the boundaries coming.
"There are a lot of learnings for us, but this was one of those days where we knew we weren’t good enough."
Northants didn't help themselves with a few dropped catches throughout the day, with Cooke benefitting a couple of times on his way to 133 not out, while Carlson, who finished on 170 not out, was dropped in his 50s.
"It’s been the story of our season in terms of the catches we have put down," said Liddle.
"There have been far too many of them, but we are working hard on our catching.
"I don’t know if Glamorgan are going to declare or not but hopefully we are going to bat out the final day.”
Carlson and Cooke's unbroken partnership of 307 is a Glamorgan club record for the fifth wicket. and has given the Welsh county a great chance of wrapping up a victory in the final Group Three game of the summer.
"Me and Chris knew there would be runs there for us if we batted patiently," said Carlson.
"Northamptonshire bowled well in parts but we weathered the storm and reaped our rewards.
"It’s great to be in the record books and it’s lovely for me and Chris to be there.
"We will have to see what the best route to winning the game is in the morning, but we will have to bowl well and take our chances.
"Northamptonshire are a bowler light and dropped some catches, and with our three big quicks there’s every chance we can take the nine wickets we need.”