Miriam Callahan remembers the patient who clarified her decision to become a political activist. He was homeless, suffered from severe arthritis in his hip and was self-medicating with fistfuls of Advil. That gave him a bleeding gastric ulcer that landed him in the emergency room at a public hospital. Callahan, who is a medical student at Columbia University, and her colleagues patched him up and sent him back to the shelter, where he began self–medicating once again. He was stuck in a horrific cycle. Arthritis isn’t a disease that should kill people, Callahan says, but in this case, it was becoming a real possibility. “It’s immoral,” she says, “the way that we treat people in this country.”

In the months since seeing that patient, Callahan has channeled her frustration into political organizing—and she’s hardly alone among her fellow medical professionals. With roughly 27.5 million Americans uninsured and nearly 80 million struggling with medical bills, doctors, nurses, medical students and other patient-facing professionals are finding themselves on the front lines of a broken system. Like Callahan, many are looking for ways to fix it. The result is that the medical field, which was once one of the most conservative professions, is becoming an unlikely hotbed of progressive political activity. One of these advocates’ top goals? Single-payer health care, now known most often by its politically charged nickname: Medicare for All.

“I don’t think I can just be a patient advocate at the bedside,” says Deb Quinto, a 38-year-old nurse in California who has canvassed in support of Medicare for All. “It’s our job to protect our community and to protect any threat to their health.”

Single-payer health care was once considered a fringe idea in the U.S. But so were the ideas that led to Medicare and Medicaid, through which the government pays for qualifying citizens’ medically necessary services. And over the course of the past few years, proposals for a universal single-payer plan have entered the mainstream political lexicon, at least that of one major party. Large majorities of Democratic voters now say they support some version of Medicare for All, and Senators Elizabeth Warren and Bernie Sanders, two of the three top-polling Democratic presidential candidates, have made the policy central to their campaigns. There are two Medicare for All bills currently pending before Congress. Medical professionals are central to this growth in popularity. From 2008 to 2017, the share of physicians who favor single-payer health care increased from 42% to 56%, according to Merritt-Hawkins, a physician-recruitment firm.

While Medicare for All remains deeply controversial among many Americans—and a nonstarter among most Republicans—physician-activists insist the tide is beginning to turn. “There’s been a sea change in the way we talk about health care reform,” says Dr. Adam Gaffney, an instructor at Harvard Medical School and president of Physicians for a National Health Program, which supports single-payer health care. He notes that as a growing number of doctors advocate for Medicare for All, the policy stands a better chance than it has in a generation. “Whatever reform we achieve,” he says, “we need them—us—to be a part of it and make it work.”

For most of the 20th century, physicians were a staunchly Republican group. Overwhelmingly white and male well into the 1990s, many ran their own practices and operated as small-business owners. Their leading trade group, the American Medical Association, reflected their members’ politics: it helped sink attempts by Presidents Franklin Roosevelt and Harry Truman to pass universal health care, hiring a public relations firm and employing doctors themselves to warn patients against national insurance. As Medicare gained steam in the 1960s, the group produced a record featuring Ronald Reagan, then an actor, to raise the specter of creeping socialism as part of its pitched, if losing, battle against the safety net for older Americans.

But in the past generation, both health care and the job of being a doctor have fundamentally changed. As the insurance industry expanded, physicians have moved from running their own private practices to being employees of hospitals and health systems. Instead of building their own patient bases, doctors nowadays often receive fixed salaries. “What that allowed physicians to do is basically look at the system in a more altruistic way,” says Travis Singleton, executive vice president of Merritt-Hawkins. “It doesn’t mean the independent physician 15 years ago didn’t care about every patient who walked in the door. They simply knew that if they didn’t control their payer mix, then they couldn’t keep the doors open.”

Meanwhile, other macroeconomic shifts have affected where doctors live, how they work and who chooses to join the profession in the first place. Beginning in earnest in the 1990s, hospitals and medical groups began consolidating, pushing once rural and suburban doctors into big cities. And as medical schools became more expensive, aspiring doctors began taking on ever larger debt loads. In 2018, medical-school graduates carried a median $200,000 in student debt, a burden heavy enough to reshape expectations. “If you want to make a lot of money, maybe go into finance or business consulting,” says Courtney Harris, a Chicago medical student, who will have $300,000 in student loans when she graduates.

As the economics of medicine have shifted, so have the underlying demographics of the profession. In the past two decades, more women and people of color have entered the profession. Medical schools, meanwhile, have expanded their curricula to include information about gun violence, climate change and how social determinants, like class and race, affect people’s health. “These are not just our patients, but our parents, our cousins, our uncles, our grandparents,” says Yoseph Aldras, a medical student whose parents are Honduran and Palestinian.

Medical schools have embraced these ideas so thoroughly that when a former associate dean at University of Pennsylvania’s medical school complained about politicized medical education in a recent Wall Street Journal op-ed, he was met with swift backlash from the school’s faculty, students and more than 150 alumni. Other schools have gone further, such as at Oregon Health and Science University, where students themselves now teach a required course on structural factors in health like institutional racism or immigration.

Singleton, whose firm conducts a biennial survey of doctors’ opinions, says that while there are myriad reasons for an uptick in political involvement, one of the most compelling is simple: doctors see the dysfunction of the health care system on a daily basis. As health care costs ballooned and the private insurance industry expanded, the job of being a doctor changed. Instead of just treating patients, doctors today must battle with insurance requirements, manage arcane reimbursement systems and juggle enormous administrative costs, Singleton’s firm found. Much of this is a direct consequence of physicians’ early opposition to health care reform, explains Beatrix Hoffman, a history professor at Northern Illinois University. By pushing back against government involvement, she says physicians created the system that is now dominated by private insurance. “We’ve heard so many horror stories from doctors who have come before us about spending hours on the phone negotiating with insurance companies,” says Scott Swartz, a 28-year-old medical student in San Francisco. “That’s not how we want to spend our time.”

All of these factors have combined to shift doctors’ politics to the left. In 1994, 67% of political campaign contributions by doctors went to Republicans, according to research by Adam Bonica, Howard Rosenthal and David Rothman. By 2004, donations to Republicans dropped below 50%. And by 2018, the ratio had more than flipped: Democrats captured more than 80% of physician donations last year. “There is an absolutely notable shift over 25 years away from Republicans,” says Rothman, a professor of social medicine at Columbia University’s Vagelos College of Physicians and Surgeons. “And it’s persisting.”

A decade ago, many physicians’ groups supported the Obama Administration’s effort to pass the Affordable Care Act, which aimed to extend access to health insurance to nearly all Americans. While the law failed to keep insurance costs low for many Americans, Republicans also failed to present a workable alternative to American voters. Though Republican lawmakers maintained control of the House and Senate in 2017, their attempts to repeal or replace the flawed Obamacare failed, leaving millions of Americans to continue to struggle with sky-high health care costs. This fruitless political maneuvering galvanized many in the physician-activist community. It was clear that Obamacare, which was designed to safeguard access to quality insurance, wasn’t doing enough, they argued. Why not push for a system that skips insurance entirely and instead offers access directly to quality care? “There’s a growing recognition among physicians that the current system, even with the ACA, costs too much, leaves too many people behind,” says Bob Doherty, senior vice president of governmental affairs and public policy at the American College of Physicians (ACP).

Enter: renewed interest in single-payer plans. In 2016, the American Academy of Family Physicians, which has supported the idea of “health care for all” since 1989, launched a study of various payment models, hoping to inform discussions of how to reform the health care system. The American College of Physicians, which supports a government-funded option for health insurance, is developing its own recommendations too.

The AMA, which has maintained its opposition to Medicare for All, began softening its rhetoric. “The AMA has and always will welcome debate at our House of Delegates on moving forward on health care reform,” says AMA president Dr. Patrice Harris. This attitude reflects a broader cultural consensus, according to Hoffman, the NIU historian. “It’s become more unacceptable than in the past to so blatantly oppose the expansion of health coverage,” she says. Bonica, who led the research on physician partisanship, says that incremental shift makes a difference. “There’s potential for physicians to organize among themselves,” he says. “Conditions are very ripe for that.”

At the AMA’s annual meeting in June, members voted on a proposal to remove the organization’s opposition to single-payer health care. It lost, but narrowly—just 47% to 53%. Outside the meeting, a group including doctors, nurses and medical students held a rally and shared stories about why they wanted to fight for universal health care. Two months later, advocates for a government-backed health care option scored another victory when the AMA pulled out of Partnership for America’s Health Care Future, the industry coalition aimed at stopping single-payer and public-option plans.

Meanwhile, more doctors are joining activist organizations. Physicians for a National Health Program, an advocacy group of doctors that has been pushing for single-payer since the 1980s, now has 23,000 members across the country and has added 14 new chapters since 2017. PNHP’s student arm, Students for a National Health Program (SNaHP), has grown rapidly as well, says Dr. Richard Bruno, who helped found SNaHP in 2011. It has nearly doubled its membership in the past three years and now has 85 chapters at campuses across the country. Peter Lorenz, a second-year student at Rosalind Franklin University’s Chicago Medical School who helped start a SNaHP chapter this fall, says the base is energized. The old guard “know things are changing,” he says. He’s now working with his school’s student chapter of the AMA, which wants to get Illinois’s state physician group to drop its opposition to single-payer health care.

It’s not just aspiring physicians joining the fight. At Columbia, the SNaHP chapter includes students studying dentistry, physical therapy and nursing. Nurses are also out in force, says Bonnie Castillo, the executive director of National Nurses United, whose members have long advocated for single-payer health care. Beginning in February, NNU knocked on 20,000 doors and held nearly 2,000 events talking to voters about Medicare for All. During the congressional recess in August, 1,200 activists organized in 49 House districts. “We’re thrilled that we have this surge of youth and of activism,” she says.

But the path forward is uphill. Part of the struggle, PNHP’s Gaffney says, is educating people about what health care reform actually means. Aside from repealing the ACA, Republicans have not offered a coherent plan for the future of health care, but most of the Democratic presidential candidates are vague on details of how their health care proposals would work too. Whether the public supports Medicare for All depends on how pollsters describe the policy. Some universal health care proposals would eliminate all private insurance while others would offer voters the option of choosing to access government health care. In the meantime, medical students, doctors and nurses are still debating what exactly the nuts and bolts of an ideal policy would be.

Callahan, the medical student at Columbia, sees education as central to the fight. This semester, she is creating a workshop to help her fellow medical students translate their frustrations into political action. To her, advocating for Medicare for All is, at its heart, a moral fight. Doctors and nurses are consistently ranked among the most trusted professions in the U.S.—and that, she says, comes with an obligation to reform a system that too often leaves families in bankruptcy or forces patients to forgo care that they need but can’t afford. “The idea that things have to be done a certain way because that’s the way they’ve always been done—in the Trump era, that doesn’t hold a lot of water,” says Callahan. “If we gain enough power we can actually make that change and bring about the world we want.”

Write to Abigail Abrams at abigail.abrams@time.com.