WASHINGTON — Thousands of part-time college professors are joining labor unions, a growing trend in higher education that’s boosting the ranks of organized labor and giving voice to teachers who complain about low pay and a lack of job security at some of the nation’s top universities.

The move to unionize at campuses from Georgetown University in Washington, D.C., to Tufts University near Boston follows a shift in hiring practices at colleges that rely more than ever on adjunct faculty to teach classes.

Last month, adjuncts at Tufts became the latest to join the 2.1 million-member Service Employees International Union, which has been aggressively targeting college instructors. Adjuncts at Georgetown formed a union with SEIU in May, and part-time instructors at nearby American University joined the union last year.

SEIU now represents more than 18,000 members at 10 colleges and universities, compared with 14,000 five years ago. The union is preparing to file for elections at more colleges in the Los Angeles, Seattle and Boston areas.

Adjunct professors now make up more than half of all college faculty nationwide; in the 1970s, about 70 percent of college instructors were tenured professors or on a track to tenure.

Unlike full professors, most adjuncts earn just a few thousand dollars per class, with scant benefits and little job security.

“What started out decades ago as a way to supplement experience on college campuses by using adjunct professors has flipped,” said Malini Cadambi, SEIU’s national director of higher education. “They are the majority of faculty labor on many campuses now, and their position has not improved.”

Kip Lornell, an adjunct music professor at George Washington University, has been teaching students for 25 years and is the author of 13 books on American music. He earns less than $23,000 a year teaching three classes at GWU. By contrast, a full professor at the university earns an average salary of $156,000 a year, according to data compiled by the American Association of University Professors.

Lornell says conditions have improved since GWU adjuncts formed a union in 2006 and won a contract two years later. Salaries are 20 percent higher, and the university now pays minimum rates of $3,500 or $4,030, per 3-credit course, depending on the lecturer’s degree.