WASHINGTON (AP) — 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.