The Welsh Liberal Democrats have voted unanimously to cap primary class sizes to 25 if they win power in next May's Assembly elections.
But staunch opposition to introducing statutory class sizes from Jane Davidson, minister for education, lifelong learning and skills, could place strain on a forecast post-election LabourLib Dem alliance.
Headteachers recently complained that the annual 1 per cent efficiency savings required of local authorities by the Assembly government could mean redundancies and bigger class sizes. But Ms Davidson claims teaching groups are smaller than ever, with the best-ever teacher-pupil ratios.
Lib Dem Cardiff councillors led the call for smaller class sizes at the party's annual conference in Aberyswyth last Saturday.
Delegates were told that smaller classes at primary level would improve discipline and give children a better chance in life. The Lib Dems also see it as a way of contributing to a reduction in the number of spare places, estimated at around 70,000 in Welsh schools.
But Ms Davidson has already blanked calls from the party to curb classroom sizes in both Welsh primaries and secondaries, say party leaders. She apparently refused to meet with Cardiff council leaders after members passed a motion in June to cut class sizes to a maximum of 25.
Councillor Bill Kelloway, Cardiff's member for education and lifelong learning, said then that reducing surplus places should be used as an opportunity to cut class sizes.
However, Lib Dems claim the minister refused to enter into talks at the time, stating there was no proven link between class size and pupil achievement.
The most recent Assembly government statistics reveal that most infant and junior classes in Wales contain 30 pupils or fewer.
There were 100 junior classes with more than 30 pupils last year, down from 103 in 2004. There were also no junior schools with more than 35 children.
This compares to 1999, when a quarter of junior classes in Welsh schools exceeded 30 pupils.
The number of infant classes with more than 30 pupils was up slightly in 2005, from 38 to 46. In total, 4,567 primary-age pupils were in over-size classes in September 2005.
The Assembly government has provided pound;75 million over the past five years for class size reduction.
But Peter Black, the Lib Dem chair of the Assembly's education committee, said: "Reducing class sizes further will help teachers to focus on teaching and children to focus on learning - a win-win situation."
Rodney Berman, leader of Cardiff council, said many local authorities wished to cut class sizes but the Assembly government's funding arrangements made it impossible. He called for an overhaul of the system.
As TES Cymru went to press, the council leader was facing a vote of no confidence over a projected pound;7m overspend in adult social services.
Earlier this week, Mr Berman blamed the Assembly government for not providing sufficient funding.