If the Chicago Teachers Union loses its fight with the district, it could have ripple effects around the country. (Jose M. Osorio/Chicago Tribune/MCT)

The massive teacher strike in Chicago offers a high-profile test for the nation’s teacher unions, which have seen their political influence threatened as a growing education reform movement seeks to expand charter schools, get private companies involved with failing schools, and link teacher evaluations to student test scores.

The unions are taking a major stand on teacher evaluations, one of the key issues in the Chicago dispute. If they lose there, it could have ripple effects around the country.

Both the National Education Association and the American Federation of Teachers are “a bit weaker,” said Rick Hess of the American Enterprise Institute, a conservative-leaning think tank. “They are playing on more hostile terrain, and they are facing opponents the likes of which they have not had to face before.”

Members of the Chicago Teachers Union—the AFT’s oldest local—walked off the job Sept. 10 for the first time in 25 years over issues that include pay raises, classroom conditions, job security, and teacher evaluations.

They are pitted against Chicago Mayor Rahm Emanuel, a powerful Democrat—and former chief of staff to President Barack Obama—who wants to extract more concessions from teachers while the school district faces a nearly $700 million deficit.

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Major teacher strikes have been rare in recent years, compared with the 1960s and 1970s, when teachers went on strike frequently for better pay and improved bargaining rights. While unions generally got what they wanted in the past, they face a tougher climate today.

With the weak economy, unions have seen massive teacher layoffs, increased class sizes, and school districts unable or unwilling to boost teacher salaries. Like other public employee unions, they are also under attack from Republican governors like Scott Walker of Wisconsin, who signed a measure last year to curb collective bargaining rights and limit benefits for state workers.

The 2.2-million member NEA has lost more than 100,000 members since 2010, as fewer public school teachers are hired and more charter schools open, most of which are not unionized. At the 1.5 million-member AFT, years of steady growth have leveled off.

“They certainly are on the defensive,” said Richard Ingersoll, a professor of education and sociology at the University of Pennsylvania. “They are under attack. A lot of times they are demonized. On the other hand, there’s really smart and progressive elements in the teacher’s movement who want to get out ahead of [school reform issues] and do it in a way that’s fair.”