Chicago Teachers Union vice president Jesse Sharkey has probably thought long and hard about whether it's a good idea for his union to go on strike Friday; the risks are high. But in hearing him talk about it last week, you couldn't tell.

"We are going to strike over things that judges might consider illegal, but we consider moral and right," he said at a public-sector union conference in New York. "There might be judges that disagree with us..." He shrugged. "But we disagree with them."

The Chicago Teachers Union will be striking for the second time in the past four years on Friday. But this isn't your average strike — it's rife with political demands that go beyond typical union demands, it looks a lot like the type of strike that European or Latin Americans workers often engage in but that is very rare in the US, and it may be illegal.

The teachers insist that 27,000 of them will be walking off the job because of a broad commitment to public schools and the public sector as a whole, fighting not just for a contract for themselves but for all poor and working-class people in Chicago. Chicago Public Schools officials insist the strike is illegal and harmful to the city's children.

Along with more than 40 community groups and several other unions at nearly three dozen scheduled protests and rallies across the city, the union's principal demand is that the city and state raise revenue by taxing the rich.

"Black and brown children on the South and West sides are bearing the brunt of this trauma," CTU President Karen Lewis recently said, referring to the numerous budget cuts and layoffs the school district has recently enacted. "We cannot go on like this."

To understand today's strike, you have to first understand the union's recent internal shifts. In 2010, a new leftist, militant leadership called the Caucus of Rank-and-File Educators won election in the union; two years later, those leaders took the union out on strike for the first time in a quarter century.

The strike was notable for multiple reasons: strikes are near an all-time low in the United States these days; teachers were up against Mayor Rahm Emanuel, a power player in the national Democratic Party with a track record of business-friendly, anti-union policies; a national political climate had turned against teachers, making the idea of a strike seem unwise.

But perhaps most importantly, the teachers successfully made a case to Chicago's public that they were fighting not just for themselves, but for the city's schoolchildren — and that they were using a strike to do so. Multiple polls showed that parents believed them, siding with the union over Mayor Emanuel.

The union has maintained that message in the years since the strike, and parents appear to still believe them: a Chicago Tribune poll from February found that Chicagoans' support for the union was still strong, with three times as much support in the city for the CTU than Mayor Emanuel.

In 2012, however, the strike itself was clearly legal: the union and the district had come to loggerheads over contract negotiations, and the multiple legal hoops the union had to jump through to walk off the job had all been fulfilled.

On Friday, the union is in riskier territory. The teachers' legal justification for striking stems from one specific issue, the unilateral elimination of the "steps and lanes" provision of their contract by the Board of Education. Typically, when a union contract expires, workers continue to work under the terms of that contract until a new one can be negotiated.

"This may or may not be a legal strike. This is uncharted territory."

The previous contract included steps and lanes to pay increases ("steps" related to the number of years spent teaching, "lanes" related to new teaching credentials like master's degrees). Yet last fall, the district decided to stop giving such raises. The union says this qualifies as an unfair labor practice and is a strikeable offense.

CPS CEO Forrest Claypool disagrees. "The law means what the law says, and this is an illegal strike," he said recently.

Bob Bruno, professor and director of the University of Illinois Labor Education Program, says it isn't clear what the law says on the walkoff's legality. Established law on unfair labor practices has mostly to do with the private sector, and teachers are public sector workers.

"This may or may not be a legal strike," Bruno says. "This is uncharted territory."

Few unions take the risk of illegal strikes, and those that do sometimes pay a serious price. In 2005, New York City transit workers illegally walked off the job. The union was fined millions of dollars, its president was briefly sent to jail, and automatic union dues deductions from members' checks were ended.

But in addition to the CTU being more tightly organized than the transit workers (86 percent of all the union's members voted in favor of a strike in December 2015, and CORE made strong membership engagement a top priority after taking office), Bruno says the transit workers knew they were clearly breaking the law. The CTU's legal situation is murkier.

If the district were to bring charges against the union for the strike, they would be heard by a state labor board — one with several members appointed by Republican Gov. Bruce Rauner, whose opposition to public sector unions has been the hallmark of his time in office. (In 2012, before he was governor, Rauner emphasized he did not believe public sector unions should exist, and refused to say whether or not he believed the same for private sector unions.)

But issues of legality are besides the point, Bruno says. "They're not determining whether they strike or not on the basis of their employers or educational labor relations board," he says. "There are bigger issues at play here. They're trying to change some minds."

The union is walking a fine line between the narrow issues they are legally permitted to strike over and those "bigger issues."

"This [strike] is a call for revenue for funding the schools and social services in this state appropriately," CTU President Karen Lewis recently told Chicago Tonight, shortly after explaining they were striking over the "steps and lanes."

The union says that school closings and round after round of budget cuts and teacher layoffs have meant that many schools aren't able to accomplish their most basic tasks.

"We're not able to function with this low level of funding," says Sarah Chambers, a special education teacher at Saucedo Academy. "And the board says they're going to make more cuts."

The strike comes amid a longstanding budget battle between Illinois's Democratic-controlled State House and Senate, and Gov. Rauner. A former private equity mogul and near-billionaire, Rauner has refused to pass a budget for the state without new rules restricting public sector workers' union rights and has enacted deep budget cuts that have caused numerous social service agencies in the state to close down or drastically reduce services. Illinois is currently the only state in America without a budget.

The union's demands for increased revenue — a tax on millionaires, a tax on financial transactions like futures and options trades, and a progressive state income tax (Illinois is one of the few states that has a flat income tax) — can't be won in contract negotiations. Some would require state constitutional changes. That makes a union victory hard to define.

"Victory will be showing a united force — not just teachers and parents and students, but actually creating a movement with other workers from around the city and the state," Chambers says.

Still, the fact that an American union is going on strike alongside other unions and community groups with broad political demands is almost unheard of.

"[Such strikes] happen pretty much everywhere but the US," says Professor Bruno. "They're very common in France, they're common in Germany and Central and South America. It's only in the US, because of the historical evolution of labor law, that you can only strike legally under the narrowest of conditions. And a political strike over larger policy issues is clearly prohibited."

That makes today's strike "extraordinary."

The action "hearkens back to the '30s and '40s, when organized labor was using the strike to make larger economic and political points and trying to pursue broader economic and social goals," Bruno says. "We don't have much precedent for it."