On the last day of classes this semester, graduate students at Harvard University went on strike over pay, benefits and the handling of harassment and discrimination complaints.

"Hey Harvard, you can’t hide, we can see your greedy side," hundreds of students chanted while walking the picket line on a recent snowy morning in Harvard Yard.

For more than a year, the Harvard Graduate Student Union has been negotiating with administrators, failing to reach an agreement on their first contract. Cresa Pugh, a fourth-year doctoral student in sociology, called striking "kind of the last resort that we had."

Pugh, who teaches a course on the history of the Caribbean, said one thing the union — which is affiliated with the United Auto Workers — is fighting for is affordable health care. She noted that full-time faculty members pay about $20 to $50 a month for dental care, while graduate students like her pay hundreds of dollars a month.

"My first year here I didn't have dental insurance, and I had to have a root canal, and it was $2,000 out-of-pocket and time that I had to take off,” Pugh said. “It's really unaffordable for someone that's just on a stipend."

A Harvard spokesperson tells WGBH News administrators are ready to bargain with the students. The two sides are now scheduled to meet next Wednesday.

Still, striking graduate students say they are frustrated by what they see as Harvard’s lack of urgency to address their concerns.

“While we are glad to hear we have a new bargaining session as a result of our organizing, if Harvard were serious about reaching an agreement, they would be at the table today and every day until a contract is signed,” said Cherrie Bucknor, a PhD candidate in sociology.

The first public graduate student union in the country was formed in 1966 at the University of Wisconsin-Madison. In the 1970s, job prospects for PhD students stagnated and student loan debt had swelled. Over the past 50 years, unionization has grown in popularity, although organizing attempts haven't always been successful.

In 2016, the National Labor Relations Board under the Obama administration ruled that graduate students at Columbia University and other private colleges could collectively bargain. Four of the nine graduate student unions that have organized since then are here in the Boston area — at Harvard, Tufts, Brandeis and Boston College.

Boston College is one of at least 20 public and private universities across the country with graduate unions that have not been recognized. Students at the college filed a petition to form a union in 2017, but the NLRB has not recognized it.

Grad students at Brandeis say they've seen as much as a 56 percent wage increase over a three-year agreement reached in 2018. The Tufts union also scored some wins in its first contract on a campus with a progressive streak.

"The faculty I work with are very pro-us,” said Paul Driskill, a third-year PhD candidate at Tufts who teaches English composition.

Sitting in a campus café between classes, Driskill said organizing has improved graduate students’ benefits, including 12 weeks of parental leave. Since the union and administration began negotiation in 2017, they’ve also seen their base pay go up by nearly $2,000 a year.

“In order to differentiate the folks who are assisting rather than teaching a course, they’re giving us a $1,000 bonus to recognize that it’s a lot more work to actually form and build and teach a class all on our own,” Driskill said, adding that his bonus goes a long way toward paying for a room he rents.

"My apartment in Dorchester is only $550 dollars a month, so if it goes up, I get two months of rent, which is like half the summer. So it's like a huge deal," Driskill said.

All of these gains, the union said, help alleviate graduate student workers' stress and anxiety about their debt and dismal job prospects.

In a statement, James Glaser, dean of the Tufts School of Arts & Sciences, said the university’s relationship with its unionized PhD graduate students is continuing to work well, and it has collaborated with the union towards the effective implementation of the contract.

What’s driving the increase in organizing at private colleges?

"We've developed this absolute glut of people with doctoral-level training who have nowhere to take that training," said Tom DePaola, a fellow in urban education policy at the University of Southern California and a member of USC's Graduate Student Organizing Committee.

DePaola’s research finds as tenure-track job openings for professors decline or flat-line in most fields, colleges are increasingly tapping graduate students to pick up the slack and students are organizing unions to resist what he calls "the gig academy."

"The gig academy is a way to conceptualize how the gig economy has really been able to grow as a model inside of higher education," DePaola said.

Just as Uber, Lyft and Amazon hire workers on independent contracts with low pay, few benefits and little job security, graduate students find themselves in a similar situation.

"It is hurting the quality of teaching and learning, and it's hurting campus cohesion," DePaola said. “It's causing lots of mental health issues in staff, faculty, students and grad students, especially."

Research shows mental health struggles are much more common among graduate students than among the general population. A Harvard study published in November of last year found graduate students who struggled financially were two to three times more likely to exhibit anxiety and depression compared to those who did not.

DePaula said unionized graduate workers are at the forefront of moving the labor struggle beyond individual silos on campus, showing solidarity with local immigration and affordable housing groups.

"They've really taken the lead in trying to build this broad-based social movement," he said.

WGBH Radio's Marilyn Schairer contributed to this report.