There’s a principle of negotiations that if what one wants is half a loaf one should ask for the whole loaf. If one asks for only half a loaf at the start, one is apt to have to settle for a quarter of a loaf, if that. Which is precisely the predicament Mayor Bloomberg and Chancellor Klein are facing in respect of charter schools, following the budget compromise in Albany. The pain of that recognition — that the mayor asked for half a loaf and got even less, that he should have been shrewder from the outset — no doubt contributes to the anger to which the mayor gave vent in his attack yesterday on the opponents of charter schools.

What set the mayor off, our Sarah Garland reported, was a question about a new requirement that charter schools hire unionized teachers if enrollment reaches 250 within two years. “It is,” the mayor shot back, “a disgrace that when you have such demand that there’s anybody, at any level of government, who’s trying to limit parents’ options, particularly given the success of most charter schools.” He then promised, Ms. Garland reported, to publicly condemn each individual lawmaker at the state and local level who had tried to limit charter schools, which are funded with public money but operate largely outside the education bureaucracy.

“For reasons non-pedagogical, people are trying to limit parents’ choices,” the mayor went on, “These are the same people who stand up repeatedly and say, ‘I’m for parents; I’m to help protect my constituents.’ … If they’re not standing up for parents, I’m just going to point it out to everybody.” Our Ms. Garland pointed out that the increase in the cap comes as some charter school administrators are reporting a “growing gap between a large number of applicants and a limited availability of seats this spring.” Some reports indicate that the waiting list to get into charter schools is almost as great as the number of students — 15,000 — in the charters schools New York City is allowed.

It’s encouraging to see the mayor starting to put some passion into this struggle, all the more so because he and the chancellor went into the fight with the legislature eschewing a full campaign on principle. That would have meant a campaign for vouchers, tuition tax credits and deductions, and parental choice. Instead, the mayor kept whinging that vouchers aren’t a realistic solution for more than a handful of students. He contented himself with half a loaf — an increase in the cap on charters. What they got was a quarter loaf — nay, a thin slice. The 50 or so added charters the city is going to get will provide a choice to only a tiny percentage of the parents who wanted a way to take their children out of our public school monopoly system.

Since the mayor is going to make only a start on school reform in his eight years in City Hall, this is a moment for him to start thinking about his education strategy on the national stage — whether he runs for president, which we’d like to see him do, or tinker from the sidelines through his philanthropic foundation. The strategic point is that the way to move this issue is through a national campaign — in the courts, in the legislatures, and in the executive branch — for the principle of parental choice, which means vouchers and tuition tax credits. There is no point mousing around with a focus only on charters. They are wonderful institutions, but they leave parents at the mercy of legislatures and the teachers unions. And leave everyone from the mayor to the parents hopping mad.