An advocacy group known for criticizing the excesses of “union bosses” claims the Chicago Tribune violated its free-speech rights by rejecting an ad targeting the Chicago Teachers Union.

The Chicago Tribune refused to run a full-page ad from the Center for Union Facts comparing the union to former Alabama Gov. George Wallace. The group suggested the newspaper cited a “racial undertone” as its reason for refusing to sell it ad space.

“Someone New is Standing in the School House Door,” the ad blares: “Teacher Unions.”

It pictures George Wallace standing in a doorway at the University of Alabama, barring two black students from entering the school on the first day it was integrated. Underneath the image are two words in bold: .

The group’s ad says the union whose strike has paralyzed Chicago’s schools — and others like it — “bully school officials into keeping bad teachers, scare politicians who support school reform, and block efforts to fix failing schools.”

USA Today, based far away from the Windy City, ran the ad Friday.

“For the Tribune to refuse to run our advertisement strikes me as cowardly, biased, and frankly, a violation of free speech,” said Center for Union Facts executive director Rick Berman in a statement.

“To say that the ad has ‘racial undertone’ is to miss the point entirely.”

The Chicago Teachers Union struck Monday over job security for teachers who have been on the job the longest, the inclusion of’ standardized test scores in teacher evaluations, salary increases and other contract issues. Chicago is home to the nation’s third-largest school district.

Center for Union Facts managing director J. Justin Wilson countered that the strike was more about “protecting bad teachers, maintaining the status quo, and preserving the union’s power.”

The center is funded by industry sources and individuals. It does not disclose the identities of its donors.