The most memorable events have occurred in Wisconsin, Indiana and Ohio (where the unions came out on top). Even lefty Illinois went ahead with legislation that made it more difficult for teachers to strike. And now Michigan, a state seemingly built by blue-collar workers, has gotten in on the action. Last night the governor signed "right-to-work" (RTW) legislation passed in the dying days of a lame-duck session of the state legislature.

RTW legislation prevents unions from compelling workers to pay union dues, though it does not prevent them from organising. Liberals and labour organisations say this creates a free-rider problem, as non-union members benefit all the same from union-negotiated contracts. Conservatives and business types counter that employers prefer states with RTW legislation, so it creates more jobs. Politifact looked at the numbers and found that 15 of the then 22 RTW states were among the top 25 in job growth between 2001 and 2011. Of course, other factors affect those statistics too.

Kristin Dziczek, the director of the labour and industry group at the Centre for Automotive Research, says RTW is rarely the deciding factor in relocation decisions by employers. Firms are more focused on being close to customers, suppliers, transportation routes and a skilled populace. Incentives also matter, as does the right piece of land. Somewhere below all this comes RTW laws, says Ms Dziczek.

The president, for his part, has called RTW the “right to work for less money”. But it is not clear whether RTW legislation depresses wages in a state. Some left-leaning groups say that it does. But it is difficult to pick apart the different causes of lower wages, such as minimum-wage levels, educational factors and the skill-level of a workforce.

Rick Snyder, Michigan's seemingly moderate governor, has a different take. Mr Snyder claims that "had Michigan adopted a right-to-work law in 1977, per capita income for a family of four would have been $13,556 higher by 2008." He says the argument finally came to a head because of a recent attempt to get pro-union legislation added to the state’s constitution. But far from inviting employers to Michigan, the legislative chaos that is likely to result from RTW may actually make the state less attractive.

It is hard to overstate just how angry union members in blue-collar Michigan are about RTW. Only a few years ago autoworkers gave up many hard-won benefits in order to save the ailing car industry. Those workers (whose contract expires in 2015), along with thousands of other union members, arrived yesterday in Lansing, the state capital, to protest the legislation. Leaders promised “class warfare” and a battle at the ballot box. They even threatened to turn up at the soccer games of the governor’s daughter.

The real villains, in their opinion, are the unaccountable plutocrats, such as the Koch brothers, who are pushing the anti-union agenda with the help of right-wing legislative groups such as the American Legislative Exchange Council. These groups, though, are pushing on an open door. The cities and states of the Midwest are struggling with deficits, pension crises and unemployment. Curbing the unions has seemed like a proper solution to some officials.

National unions now face a dilemma. To what extent do they open up another expensive front in the fight over labour? Michigan is shaping up to be a long battle that will likely drag through the courts. Some national leaders, such as Randy Weingarten of the American Federation of Teachers, believe the Michigan fight should be kept at the state level. But other leaders, such Harold Schaitberger of the International Association of Fire Fighters, have already turned up to fight.