FROM a block away, the striking teachers camped out around Arizona’s capitol at first looked like a solid sea of red, the colour of their T-shirts and tents. On closer inspection, they distinguished themselves the way the teachers have always distinguished their classrooms—with handmade signs. Leah Falcon (“Arizona exports: Cotton, copper, teachers”), who teaches middle-school maths, said she was “fighting because my kids deserve better than 34 students in a class.” Megan Marohn (“Arizona Spending per Student: $9,000. Per Inmate: $24,000”) is a classroom aide and lifelong Republican who frets that Arizona’s Republican legislature and governor “put the value of corporations above students”. Jay Bertelsen (“Christian Non-Union Conservative Teacher Fighting for Funding”) has taught computer science outside Tucson for 25 years; his children qualify for Arizona’s state-subsidised health care for poor families.

Grievances such as these have motivated teacher strikes in five states. They look likely to continue—galvanising public-sector workers in states where Democrats hope to make gains in this autumn’s midterm elections.

The strikes began on February 22nd, when teachers in West Virginia walked out. Two weeks later the state’s Republican governor gave them a 5% pay rise—bringing the average high-school teacher’s salary to just under $48,000. Kentucky’s legislature approved a sizeable increase after teachers there walked out. A threatened strike in Oklahoma prompted legislators to boost education funding and teacher salaries (the teachers struck for nine days anyway). Last week teachers in Colorado and Arizona walked out to protest against low salaries and stagnant funding.

Those conditions are widespread. The average American teacher earns less than 60% of what a similarly educated professional makes. In inflation-adjusted terms, teachers’ salaries have fallen by 1.6% over the past two decades. But the acute crisis in public education dates back to the recession of 2008, which hit many states’ property-dependent tax receipts.

Most states cut school funding; in some, it has yet to return to its level before 2008. In inflation-adjusted terms, teacher salaries are almost 5% lower than they were a decade ago, even as teachers’ retirement contributions and health-insurance premiums have gone up. Some teachers even pay from their own pockets for classroom supplies that state funding fails to cover.

And some states have continued to cut taxes and education spending. According to Michael Hansen of the Brookings Institution, school funding in Arizona remains 35% below pre-recession levels. So the offer from Doug Ducey, Arizona’s governor, of a 20% salary increase by 2020 and a funding increase until 2024 has left teachers nonplussed. Joe Thomas, who heads the Arizona Education Association, the state’s main teachers’ association, wants not just more money but a new dedicated revenue stream. That is a hard sell in Arizona, home to waves of tax refugees from California and pensioners reluctant to spend their fixed incomes on other people’s children.

North Carolina may be the next domino; teachers there plan to demonstrate in the capital on May 16th, when the state’s legislature convenes. As in most of the other strike states, unions in North Carolina have weak collective bargaining powers. Some suggest that this explains low levels of pay; if states were forced to bargain with teachers’ unions, they would pay them more. But a new paper from Agustina Paglayan, a political scientist at the University of California, San Diego, suggests that this formulation is the wrong way round. Teachers gained good collective-bargaining rights in states that already paid them relatively well. Collective bargaining did not lead to increased salaries or funding.

The result, paradoxically, is that states where teaching unions are weaker now have more politically active teachers. Ms Marohn, one of the demonstrators in Phoenix, says that when parents ask her mother, also a teacher, what they can do to help, she tells them to vote. That should worry Republicans. There are 3.2m public-school teachers in America. Giving them a financial reason to head to the polls could spell trouble for some Republicans running in states with teacher unrest. Arizona, North Carolina and Colorado are all battleground states. Republicans had also fancied that they could flip the West Virginia Senate seat held by Joe Manchin, a conservative Democrat. For want of more chalk could the Senate be lost.

Correction (April 10th, 2018): A previous version of this piece identified Agustina Paglayan as a political scientist at the University of San Diego. She is in fact with the University of California, San Diego. This has been updated. Apologies.