FORTUNE — On Tuesday, faculty members at the University of Illinois at Chicago went on strike for the first time ever as part of a two-day walkout aimed at securing a union contract and increased wages.
The union, which represents 1,150 tenured and non-tenured professors, won certification in 2012, but it has yet to agree with the university on its first contract. The dispute between the union and the university is the latest episode in an ongoing effort by unions to gain influence on America’s college campuses.
The Service Employees International Union has been aggressively recruiting academics, and it recently launched campaigns to unionize non-tenured professors at Seattle University, Loyola Marymount, and University of La Verne. It currently represents more than 18,000 adjunct faculty members, up from 14,000 five years ago. The American Federation of Teachers claims to be the largest union for adjunct professors, with 70,000 non-tenure-track professors in its ranks, up by 10,000 from five years ago. The AFT represents 200,000 higher education professionals in total.
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The growing number of faculty with union representation reflects “the change in the professional positions at universities,” says Risa Lieberwitz, professor of labor and employment law at Cornell University’s School of Industrial and Labor Relations. Universities have become “more corporate in the way they structure themselves,” she says, which has resulted in fewer permanent tenure-track positions and more inexpensive, flexible non-tenure-track instructors.
According to a report from the American Association of University Professors on the 2012-2013 school year, 76% of all higher education instructional positions are filled on a contingent basis, with tenured and tenure-track full-time positions making up less than 25% of all appointments. From 1975 to 2011, the number of part-time or adjunct faculty appointments has increased by 300%.
The median pay per course for adjunct professors, who are typically paid based on their course load, totals $2,700, according to a report by the Coalition of Academic Workforce that examined per-course pay in 2010. Over 80% of adjunct professors teach part-time for more than three years; over 50% do so for more than six years, according to the CAW.
Pay is what drove UIC faculty to the picket line. They’re demanding a 4.5% merit-based salary increase this year; the university administration has offered 3.25%. The union also wants the minimum annual salary for non-tenured, full-time lecturers to increase from $30,000 to $45,000. The administration’s offer pegs that floor at $36,000.
“As ranks of non-tenured professors grow and job insecurity increases — combined with the fact that people in those positions have low salaries and very poor working conditions — it’s logical for non-tenured professors to say, ‘What can we do collectively?’” says Lieberwitz.
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Academic labor unions have made some progress. The CAW found that unionized adjuncts earned 25% more per course than those who didn’t have union representation.
Up until now, much of the academic labor movement has been for the sake of non-tenured professors. That’s because tenured professors are often better paid and receive more generous benefits. But another factor is a 1980s Supreme Court decision, National Labor Relations Board v. Yeshiva University, that found that tenured professors at private universities are considered managers and are therefore barred from unionizing. The NLRB is currently considering two cases — one dealing with Point Park University, the other related to Pacific Lutheran University — that could change that rule and open the flood gates for more professors to pursue union representation.