A sizeable difficulty;Opinion
Conservative leader William Hague has appointed one of his brightest prospects, David Willetts, as Shadow Education Secretary - to harry Labour on the issue.
Meanwhile local government leaders, charged with implementing the policy, are insisting it can only be delivered by constraining parental choice.
Their arguments are underpinned by a Coopers amp; Lybrand report, which looks at how LEAs are proposing to put the policy into effect. While many plan to create extra capacity in schools with oversize classes and employ more teachers, others are considering cutting admissions, adjusting catchment areas or transferring pupils to other schools.
Any blueprint which reduces parental choice, however, is likely to get short shrift from ministers - as Stephen Byers, the Blairite school standards minister, has already made plain.
According to Coopers amp; Lybrand, cramming more classrooms into popular schools is one solution likely to find favour with ministers. Another is to create mixed- age classes to mop up extra pupils. Such options may be questionable, but have the benefit of being cheap.
And that is the point. It is now clear that Labour's pre-election promise to fund its class size commitment by scrapping the Assisted Places Scheme will not produce enough money to do the job. If the pledge is to be honoured without rowing back on parental choice, then the final bill will be substantial.
Much will depend on whether David Blunkett succeeds in his bid to secure an extra pound;9 billion from the Treasury. If so, the indication is that a key priority will be to tackle pupil-staff ratios across the whole system, probably through restructuring teachers' duties.
The manifesto pledge was a piece of political theatre. But it is popular with both parents and teachers. If Mr Blunkett can now use it as a lever to win extra money for education, so much the better.