Normally, Dr. Peter Slavin spends his time doing what presidents of huge hospitals do: budgets, meetings, fundraising. But on Saturday, the president of Massachusetts General Hospital takes to the ramparts, speaking to the hospital's "March For Science" contingent on its Bulfinch lawn. The main "March For Science" rally in Boston takes place on Boston Common in the afternoon.

Hospitals normally steer clear of politics, but Dr. Slavin says he's moved to speak out by the threat he sees to the hospital's mission in the Trump administration's proposal to slash the federal medical research budget. It's only a proposal at this point, but it's a bad signal for Mass. General, which gets more federal research money than any other hospital.

"For 200 years, we've been trying to use research to improve the care we deliver to our patients," Slavin says. He says the Trump administration's proposal to slash the federal research budget "would be such a blow to our efforts and a blow to the hopes of the people we're trying to serve," that speaking against it now means "standing up for the values and the purpose of this organization."

Among the leaders of the most prestigious and powerful medical institutions in Boston, Dr. Slavin is far from alone in publicly endorsing the Boston branch of Saturday's national March for Science.

Dr. George Daley, the dean of Harvard Medical School, is slated to speak at a pre-march rally in the Longwood Medical Area, and again at the general gathering on the Boston Common.

Like Dr. Slavin, Daley says medical and research institutions overcome their usual aversion to politics when they see a threat to their core missions. There's a sense, he says, that "if we don't stand up, we're losing our souls."

Daley sees the march not as opposition to a particular administration or party, he says, but to specific policies, such as the proposed "crushing" budget cuts and to ideology that denies scientific evidence on issues from vaccines to climate change.

Leaders at Boston University, the Dana-Farber Cancer Institute, Beth Israel Deaconess Medical Center, Brigham and Women's Hospital and elsewhere are also publicly supporting the march, sending out information to their communities.

Organizers are predicting that the Boston march will attract tens of thousands of people, possibly second in size only to the central national march in Washington, D.C. The Facebook group for Boston's march has more than 26,000 members with thousands signed up to attend. Along with the main event on the Boston Common, it includes sub-gatherings of groups including anthropologists, librarians and Kendall Square techies.

Some in Boston-area science and medicine question the wisdom of the march as a tactic. Arthur Lambert, a cancer researcher at MIT's Whitehead Institute, says he supports strong science but warns that a protest march by scientists poses a risk: "It could be used, by politicians who would like to reject science and reject scientific knowledge, as ammunition to attack scientists as being a partisan group."

Some opponents of the march also say that researchers who complain about budget cuts look like just one more group that wants more federal money.

But Dr. Jason Wasfy, a Massachusetts General Hospital cardiologist and researcher, says the motivation for the march goes far deeper than money. It's "more visceral than that," he says.

"This isn't about the money," Dr. Wasfy says. "Every day, people like me not only try to discover new knowledge to help patients but we take care of patients," he says. "And we understand what it means to tell a 50-year-old patient's daughter or son that they're not going to make it. Scientific research is what minimizes that."

The march, Dr. Wasfy says, "is not about politics so much as it's about really hammering home that science matters. And in biomedical science, it matters for our patients' lives."