Heads and teachers are threatening to boycott next year's Sats
It is beginning to look like a slow motion car crash.
The government and primary schools are on a collision course over the national school tests in England.
What is more, the crunch could come just as the country is going to the polls in the next General Election.
In a surprising move this week, the National Association of Head Teachers decided to join the National Union of Teachers in taking the first steps towards a joint boycott of next year's tests.
Between them, the two unions represent the vast majority of primary school leaders and classroom teachers.
If they go ahead with combined action it would be a powerful threat to the government's testing programme.
It looks set to be a game of chicken between the government and primary schools.
The really interesting question is this: Whose side will parents take?
While the crunch point remains a little way off, as there is no threat to this May's tests, there is no sign of a compromise.
The unions have given the government until this summer to withdraw the current tests at ages 7 and 11.
If it were the NUT acting alone, ministers would have less to fear. After all, its annual conference regularly votes for industrial action but, almost as often, fails to win backing from its wider membership.
But if teachers feel their bosses are with them, they are likely to be bolder. And, so far, the NAHT's national council is unanimously behind the boycott proposal.
The NAHT leadership feels it has no other way of pursuing its campaign against the tests.
Encouraged by the government's scrapping of the national tests at age 14, it genuinely thought reform was on the way.
But subsequent ministerial comments that the Key Stage 2 tests are "here to stay" left them feeling betrayed.
The government says the tests are "not set in stone". It is awaiting the report of its expert group on assessment and will announce changes to testing in a white paper this summer.
But ministers will insist on keeping some form of objective measure of pupil and school performance. They believe parents have a right to know how their child is doing and that government has a right to measure schools' performance.
The government ought to look back to the last boycott, says Mike Baker
The only possible line of compromise would be if the government accepted something like the Scottish model, where teachers use a bank of external tests, which they set when they think pupils are ready and which they mark themselves.
But ministers in England do not like this approach as it does not allow like-for-like comparisons between schools, nor does it bring the reassurance of external marking.
So a confrontation looks very likely. The motion to be voted on at their respective annual conferences, due in April and May, is quite specific.
It would commit both unions to holding a ballot for "joint action to boycott the Key Stages 1 and 2 statutory tests for the academic year 2009/10, if the government refuses to remove them". The ballot would probably take place in the autumn.
The tests are due to take place in all English primary schools in May 2010, the last possible (and currently most likely) date for Gordon Brown to hold an election.
A confrontation with primary schools is hardly something ministers would relish during an election campaign.
Those with long memories will recall that we have been here before. The precedents are not good for government.
In 1993 a boycott of the school tests ended in victory for the teacher unions and ignominious defeat for the then Conservative Education Secretary, John Patten.
It marked the beginning of the end of his career as a Cabinet minister.
The government tried everything to prevent the boycott.
It took out national newspaper adverts seeking parental support. It appealed to teachers over the heads of their leadership. It urged head teachers and governors to enforce the law.
Finally, it tried to win over more moderate teachers by promising a review of testing, even pre-empting its conclusions by promising to slim down the tests in future years.
There were even legal attempts to stop the boycott, right up to the Appeal Court.
With the government supporting from the sidelines, the legal challenge was led by the Conservative-controlled Wandsworth Council, which tried to win an injunction to stop the NASUWT union from implementing the boycott in its area.
Wandsworth failed and the boycott was declared to be legal.
In the High Court judgement, Mr Justice Mantell ruled that the boycott was part of a legitimate trade dispute because it was "wholly or mainly about workload or working hours".
The legal decision may have been different if Wandsworth had been challenging the NUT rather than the NASUWT.
That is because the NASUWT always stressed that it was protesting about teachers' workload, not about the effect of testing on pupils. The NUT, also part of the 1993 boycott, seemed more strongly motivated by educational concerns.
If, as seems possible, the 2010 boycott is tested in the courts, the NAHT and NUT may have to prove that they are motivated by the effects of the tests on head teachers and teachers.
Otherwise it may not be considered a trade dispute.
Ministers are hoping that the talk of a boycott is just another ritual, conference-season flurry of protest.
If they are wrong - and the indications suggest they may be underestimating the strength of feeling, particularly amongst head teachers - they should remember what happened in 1993, when the boycott was almost total and the government suffered an ignominious defeat.