The high-stakes strike by 26,000 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 Obama’s own hometown, a labor stronghold. Rahm Emanuel, the Democratic mayor and Mr. 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.