THE arguments had been rumbling on for months. But negotiations between the city of Chicago and its teachers’ union finally came to an end on September 9th. Just a week after the city’s children returned from their long summer break, their teachers began their first strike in Chicago for 25 years. About 25,000 teachers have stopped work, keeping 350,000 pupils out of school.

The mayor, Rahm Emanuel, is now in an unenviable position. Improving Chicago’s disastrous school system, where four in ten children fail to graduate, is one of his main priorities. In negotiations over a new contract for teachers, his demands have been reasonable. He wants the school day lengthened to seven hours for elementary children, for teachers to work 38 weeks a year, and to be able to introduce differentiated pay. This is a mix of performance-based pay and extra pay for working in jobs that are hard to fill and for taking leadership roles. In return for all this, teachers have been offered an average salary increase of 16%, costing $320m over the next four years. Which, given the state of city finances and a deficit in the school system of $1 billion, will be a squeeze.

Over the past eight years Chicago teachers have done well, securing raises averaging 7% a year with no changes to their terms. The main sticking points now are teacher evaluations, compensation and the rehiring of teachers who have been laid off. These last two issues are the most significant hurdles (Mr Emanuel would like schools to be able to hire the best teachers, not the most recently-fired ones). But to keep the strike legal, the unions must insist that it is about nothing more than pay and benefits.

The timing could hardly be worse, politically speaking. The strike is awkward for Mr Emanuel’s former boss, Barack Obama, whom he served as chief of staff. Chicago, a stronghold for unions, is both the hometown of the president as well as the base of his re-election campaign. Mr Obama has sought exactly the same kind of reforms nationally; but union support is also crucial to his re-election, providing donations and grass-roots activists. So, cautiously, he has declined to take a position. Mr Emanuel has some ammunition left, including the prospect of school closures and increased numbers of charter schools. Introducing vouchers, though, would be too much of a hot potato even for him.