WASHINGTON – 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 reform movement seeks to expand charter schools, get private companies involved with failing schools and link teacher evaluations to student test scores.

Union leaders 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.

The American Federation of Teachers and the National Education Association — the nation's two largest teacher unions — have been playing defense in jurisdictions around the country as Republicans and Democrats alike seek greater concessions in a bid to improve ailing public schools.