Until that moment, the ending of the Assisted Places Scheme - one of the few firm pledges in Labour's programme and essential to its promised reduction in infant class sizes - had barely featured in the news, even though it could make a vital difference in key marginals.
In Portsmouth South, for instance, where the Conservatives are defending a majority of 242, the electoral arithmetic is affected by more than 600 assisted places that would be lost under Labour.
"It definitely is an issue," said Adrian Rogers, prospective Tory candidate for Exeter, defending a Conservative majority of 3,000 and 475 assisted places. "The problem is, because of the phasing proposed by the Labour party, it's not seen as an immediate threat." He fears one or two schools might close if the scheme were abolished.
At the weekend, the revelation that an administrative bungle had placed in jeopardy some 1,200 new assisted places for this autumn came as a bolt from the blue to education ministers, who were said to be "very angry". The problem was the freeze on Government business following the election announcement. This meant that prep schools joining the scheme under the new Education Act, which received its Royal Assent at the weekend, could not sign their participation agreements with the Department for Education and Employment.
But the confusion was a boon to Labour, who could accuse the Conservatives of "an enormous cock-up" on their flagship policy - and then play Sir Galahad, coming to the rescue of uncertain parents and children by promising to honour any places offered before the election.
"If there is an understanding pending the signing of one of these agreements for this September, we will honour it," said a spokesman for David Blunkett, shadow education secretary.
"But," he added, "there'll be no more after that." Voters should be in no doubt that Labour can and will abolish the scheme as soon as it comes to power, paying only for children already in the scheme to the end of their current stage of schooling. This is one pledge the party cannot afford to shed.
There are already signs that various forms of replacement schemes would spring up in the event of a Labour victory, however. Schools with established foundations, such as Nottingham High, are already making plans to offer at least some assisted places out of their own funds.
Talks have taken place between Tony Blair and Martin Stephen, head of Manchester Grammar School, about offering free places to talented children at sixth-form level. And both Labour and the Liberal Democrats have said they are happy for local education authorities to co-operate with independent schools where there are gaps in local provision.
But a Labour victory would end the Assisted Places Scheme in its current form. Parents of children starting at prep schools this autumn with assisted places would have to look to other means of support if they wished their children to continue in the independent sector for their secondary education.
TORY MARGINALS WITH HIGH NUMBERS OF ASSISTED PLACES
(Listed in order of lowest majority) Portsmouth South 610 places Batley Spen 310 Plymouth Sutton 250 City of Chester 215 Ilford South 150 Exeter 475 Brighton Pavilion 360 Coventry South 170 Taunton 460 Worcester 750 Hereford 270 places Monmouth 230 Loughborough 260 Bury North 500 Bolton West 580 Edinburgh West 225 Bedford 410 Battersea 340 B'ham Edgbaston 600 Twickenham 220 Oxford W Abingdon 365