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.