The high-stakes strike by 25,000 public-school teachers in Chicago is only the latest episode in which the nation’s teachers’ unions have been thrown on the defensive in the face of demands for far-reaching changes.

In community after community — even in major cities with strong pro-union traditions, like Los Angeles and Philadelphia — teachers’ unions have faced a push for concessions, whether it is to scrap tenure protections or to rely heavily on student test results to determine who gets a raise and who gets fired.

And now comes this high-profile showdown in President Barack Obama’s hometown, a labor stronghold. Rahm Emanuel, the Democratic mayor and Obama’s former chief of staff, is demanding a raft of concessions that are anathema to union leaders and their members. At the same time, with many teachers and their unions already viewed unfavorably by many Americans, the union is taking a gamble by engaging in a battle over changes that some education advocates believe are needed to improve the nation’s schools.

The battle underlines just how much teachers’ unions, which have provided sizable donations and many grass-roots volunteers to countless Democratic campaigns, have been thrown back on their heels in recent years.

If Emanuel wins this confrontation, he could set the table for a major setback for teachers’ unions nationwide and a potential rethinking of teachers’ enthusiasm for Democrats in this year’s elections. Advocates of sweeping education changes like Michelle Rhee, the former head of the school system in Washington, D.C., will be able to declare that if Chicago’s mighty union was willing to accept such changes, so should teachers’ union locals across the nation.

"The teachers’ unions are on the defensive on many more fronts than they used to be," said Chester E. Finn Jr., a longtime education analyst who heads the Thomas B. Fordham Institute, a conservative-leaning education policy group in Washington. "It used to be they could just fight vouchers and charter schools. But now they face this huge set of issues," not to mention budgetary pressures that have caused large-scale layoffs.

Weakening the unions’ leverage and ranks, more than 300,000 school employees have lost their jobs since the recession ended in June 2009.

The 1960s and 1970s in many ways were the heyday of teachers’ unions. Membership exploded as many states gave public-school teachers the right to bargain collectively, turning the National Education Association and the American Federation of Teachers into two of the nation’s largest unions. Together, they have 4.5 million members.

Governors and mayors rushed to do them favors, and their campaign dollars have often helped elect union-friendly school boards. But while those two giant unions still pack a punch in federal, state and local political campaigns, their winning percentage in legislative fights has fallen in recent years as frustration with low-performing schools and rising property taxes has exploded and as Republicans have won control of far more governor’s mansions and legislatures.

And two recent movies, "Waiting for ‘Superman"’ and "Won’t Back Down," have hit teachers’ unions hard, casting them as villains whose inflexibility has kept many schools and students from improving.

"Clearly the teachers’ unions are under attack and under siege," said Diane Ravitch, an education historian at New York University who often defends the unions. "There is almost this full-court press by the right against the teachers’ unions."

She said many conservative groups, like the American Legislative Exchange Council, were eager to weaken teachers’ unions because they view them as powerful political supporters of Democrats and bothersome opponents of budget cuts.

What happened in Wisconsin last year was an undeniable watershed for teachers’ unions. Gov. Scott Walker and the Republican-dominated Legislature passed a law that severely restricts the ability of public-sector unions to bargain collectively. Unions failed in their effort to oust Walker in a recall vote in June. But in November, teachers’ unions did help persuade a majority of Ohio voters to repeal a similar law.

Eager to improve Chicago’s schools, Emanuel has taken several steps — among them pressing the school board to rescind a promised 4 percent raise — and made numerous demands that have infuriated the Chicago Teachers Union. He wants student test performance to count heavily in evaluating teachers for tenure, even though the union insists that is a highly unreliable way to assess teachers. And with Emanuel intent on shuttering dozens of poorly performing schools, the union is pressing him to agree to strong provisions to reinstate teachers in other schools when theirs are closed.

Randi Weingarten, president of the American Federation of Teachers, the parent union of the striking teachers, acknowledged that her union was on the defensive. But she said it had gotten the message that it needed to do more to embrace changes to improve the nation’s schools. She has repeatedly boasted of innovative contracts like the one in New Haven, Conn., which softens tenure protections and calls for steps to improve underperforming teachers, and Cincinnati, where the union is working closely with the school board to train teachers for new, tougher core standards.

Still, Finn, the analyst, said that while there were some showcase contracts, teachers’ union locals in various cities — he pointed to Los Angeles — continued to strongly resist change.

He said the Chicago walkout would sully the image of teachers’ unions.

"It’s probably about the dumbest thing they could do from a national standpoint," he said. "It will remind everybody that teachers’ unions are about teachers, not kids."

Weingarten said the Chicago strike did not mean that the union was truculent in resisting change. Rather, she said, the walkout resulted from Emanuel’s making the teachers feel "completely disrespected."

"Everyone is focused on Chicago, but we have reached 10 different agreements in the last month," she said, pointing to San Francisco, Albuquerque, N.M., and Central Falls, R.I.

She added that the union has agreed to more flexible rules on tenure at the state level in New York, New Jersey and Connecticut.

"There’s been a big change," she said. "You have lots of teachers’ unions and their districts working together like never before."

Even so, Richard Kahlenberg, a senior fellow at the Century Foundation, a liberal-leaning research group, said not only Republicans but also many Democrats were aiming their fire at teachers’ unions.

"They’re getting very little support from some Democrats," he said. "The Obama administration has adopted a center-right position on issues like nonunion charter schools and performance pay. The places where teachers’ unions used to look for support are no longer coming through for them."