Saturday's March for Science is political, but not partisan. So say the organizers, who insist that they can walk that fine line even in an era of ideological rancor and extreme polarization.

“We’ve been asked not to make personal attacks or partisan attacks,” said honorary national co-chair Lydia Villa-Komaroff, in a teleconference with reporters. But Villa-Komaroff, a cell biologist who will be among those with two-minute speaking slots, quickly added: “This is a group of people who don’t take well being told what to do.”

The Science March, held on Earth Day, is expected to draw tens of thousands of people to the Mall, and satellite marches have been planned in more than 600 cities on six continents. The crowd will gather on Saturday near the Washington Monument for five hours of speeches and teach-ins, culminating in the march at 2 p.m. The march will follow Constitution Avenue along the north edge of the Mall to the foot of Capitol Hill. The weather forecast is a tricky one — it's not an exact science, apparently — but attendees should be prepared for rain, particularly in the afternoon.

Protest marches may be common in Washington these days, but one centered on the value of science is unprecedented. The march is part of a wave of activism in the research community. Scientists are jumping into the political fray by running for public office — such as in southern California, where geologist Jess Phoenix, a Democrat, has announced her candidacy for a congressional seat held by a Republican.

The idea for the event was spawned during a Jan. 21 conversation on Reddit, as millions of people gathered in Washington and cities around the world for the record-breaking Women's Marches. Valorie Aquino, who is working on her PhD in anthropology at the University of New Mexico, signed on as one of the march's national co-chairs shortly after. She thought she'd be able to continue her research while coordinating the event, not anticipating how quickly it would snowball.

"This March for Science organizing has consumed my last three months," Aquino said Friday afternoon. "I’m overwhelmed. I’m inspired. I’m a little terrified. I would love to take a nap but I don’t think it's going to happen."

Most mainstream science organizations — such as the American Association for the Advancement of Science, the American Geophysical Union, and the American Chemical Society — have signed on as partners of the march, despite their lack of experience in going to the barricades.

Rush Holt, head of AAAS, said there was initial hesitation about whether this was the kind of event a scientist ought to be joining but that members of his association overwhelmingly support the decision to participate.

This is not simply a reaction to President Trump's election, Holt said. Scientists have been worried for years that “evidence has been crowded out by ideology and opinion in public debate and policymaking.” Long before Trump's election, people in the scientific and academic community  raised concerns about the erosion of the value of expertise and the rise of pseudoscientific and anti-scientific notions. Science also found itself swept up into cultural and political battles; views on climate science, for example, increasingly reflect political ideology.

Mona Hanna-Attisha, the Michigan pediatrician who sounded the alarm on lead in Flint’s drinking water, is one of the march's honorary co-chairs. Her experience as a physician in Flint paved the way for her science advocacy, Hanna-Attisha told The Post. “Pediatricians care for a population that can’t speak, can’t vote,” she said, noting that doctors take an oath to protect patients from harm. “It is your role to be an advocate.”

Arthur Edelman, who studies ovarian cancer at the State University of New York’s University at Buffalo, was on the National Mall listening to the main stage sound check with his wife, Enid, on the afternoon before the march. The demonstration tents and the roughly 190 portable toilets had already been set up. (The song the keyboardist played was, naturally, “She Blinded Me with Science.”)

The 71-year-old had not marched since Vietnam, which he did then “because I didn’t want to die.” This march is different, he said. “It’s a struggle that doesn’t have to be such a struggle.” He was marching for “the air we breathe and the water we drink.” Edelman was concerned about the future of his graduate students at a time when the NIH can only fund 9 out of every 100 grant proposals submitted. “That means you’re dead in the water unless you get results,” he said. Edelman will march on Saturday, which is also his birthday.

Under the heading of “What sign should I carry?”, the march’s official website nudges participants to go geeky rather than political: “Do you have a special love for cell biology or physics? Maybe you want to proudly tell the world that vaccines have kept you healthy? Or thank the EPA for keeping your water safe? This could be the right time to declare your support for a well-funded NIH! This isn’t about any one politician — this is about science and policy, scientists and science supporters.”

The line up for the event on the Mall includes some of science and environmentalism's biggest names. Bill Nye, CEO of the Planetary Society and another honorary co-chair, will speak, as will climate scientist Michael Mann, coordinator of the first Earth Day Denis Hayes, NASA Astronaut Leland Melvin, and the heads of many science and environmental advocacy groups.

But the organizers also aim to buck stereotypes of science as stodgy, academic and dominated by older white men by selecting speakers from a broad range of ages, backgrounds and expertise. The lineup includes Taylor Richardson, a 13-year-old aspiring astronaut who raised $17,000 earlier this year to send other young girls to see the film "Hidden Figures;" YouTube star Tyler DeWitt; chemist Mary Jo Ondrechen, a member of the Mohawk Nation and chair of the board of the American Indian Science and Engineering Society; and Gallaudet University biologist Caroline Solomon, who is deaf.

YouTube star Derek Muller and the musician Questlove are slated to emcee.

Notably, astrophysicist Neil deGrasse Tyson, the most well-known living American scientist, will not be attending a science march, according to a representative. Tyson did not respond to a request to comment on why.

No politicians have been invited to participate in the march, organizers say, even as they acknowledge that this was inspired by the Women’s March on the day after Trump’s inauguration.

“Science is nonpartisan. That’s the reason that we respect it, because it aims to reduce bias. That’s why we have the scientific method. We felt very strongly that having politicians involved would skew that in some way,” Caroline Weinberg, a public health researcher and co-organizer of the march, said at the National Press Club earlier this month.

Carol Greider, a Johns Hopkins molecular biologist and Nobel laureate, said in the conference call this week that she will bring dozens of students and postdoctoral researchers to the march. “People are actually questioning whether they can even go on and have a career in science,” she said, noting the Trump administration's proposal to cut nearly a fifth of the National Institutes of Health budget. “Potentially, we will lose an entire generation of people who are now trained and have the talent and are eager to make the next breakthroughs.”

Greider said it's possible to fight for science without “labeling ourselves” as being on one partisan side or the other. That was echoed by Elias Zerhouni, former NIH head under President George W. Bush: “This is not a partisan issue. This is not one administration versus another … It's really an age-old debate between rational approaches to the universe and irrational approaches to the universe.”

Not every scientist is convinced. Arthur Lambert, a cancer researcher at the Whitehead Institute at MIT in Boston, said he was initially excited about the science march. But as the event drew closer, it seemed increasingly unlikely that it would appear to be anything but partisan.

“It’s a bad idea to align all of science against any political administration,” Lambert said. “I don’t think that's their goal … But it runs that risk, especially after such a heated election.”

Recent political developments in Washington are among the primary drivers of this march. Before he became president, Trump promoted anti-scientific theories. He tweeted in 2012, “The concept of global warming was created by and for the Chinese in order to make U.S. manufacturing noncompetitive.” He echoed the fully discredited notion that there is a link between vaccines and autism. During the presidential transition he reportedly discussed with vaccine skeptic Robert F. Kennedy Jr. the possibility of creating a vaccine safety commission.

To run the Environmental Protection Agency, he appointed Scott Pruitt, who as Oklahoma attorney general had sued the agency many times and who, during an interview in March, said he did not believe that human activity is a primary driver of the observed climate change — a statement at odds with scientific research.

The entry ban pushed in the early days of the administration, and associated rhetoric about building walls and restricting immigration, alarmed many leaders of science-related institutions that rely on the expertise of foreign nationals (at MIT, for example, 40 percent of the faculty was born outside the United States, according to the university's president).

The administration has not taken some actions initially feared by the scientific community, such as wholesale deletions of government climate data. The EPA’s website continues to describe climate change as largely driven by human activities.

Trump has yet to appoint anyone to several key science-related posts. He has not picked a White House science adviser. He hasn’t nominated anyone to run NASA or the National Oceanic and Atmospheric Administration or the U.S. Geological Survey. He has let Francis Collins stay on an interim basis as head of the NIH, though it’s not clear that Collins will be permanently retained. Public health positions are unoccupied that are crucial for responding to a global pandemic, a disaster every president since Ronald Reagan has faced.

Behind the scenes at the March for Science, there has been internal controversy about inclusiveness and diversity, and whether social justice should be central to the march’s messaging. On social media, a number of scientists have said they are skipping the march because they think the organizers haven’t focused enough on racism, sexism, and the scientific community’s centuries-long history of marginalizing women and people of color.

Meanwhile, conservative critics have derided the march as an ideological enterprise in which what’s being advocated is not science, exactly, but left-leaning policies, such as the Obama administration’s environmental regulations designed to curb carbon emissions.

It's unclear what percentage of participants in Saturday's science marches around the country will be involved in research directly. Among the non-scientists will be Dennis Moore, a patrolman at the Cherry Hill, N.J. Police Department, who said he's going to the Boston march to make the point that science benefits everyone.

“I've always been kind of a nerd,” he said, mentioning that he was wearing a flux capacitor T-shirt as he spoke. “Science is our best tool for understanding the world around us … but I think we’re basically just de-emphasizing science in our lives and in our communities.”

The first official march-related event in Washington kicked off Thursday evening in Dupont Underground, an abandoned trolley car station and former fallout shelter that is now a cavernous arts space. “Poetry x Posters” featured seven-foot-tall posters with science-themed poems. “I pledge allegiance to the soil of Turtle Island,” one poster read, “one ecosystem/in diversity/under the sun.” Four poets spoke from a stage while the audience of a few dozen listened, drinking wine from plastic cups and beer from tall cans. The poets professed a fascination with science, if not always a deep understanding.

Science and poetry both arise from the same desire for exploration, said poet Jane Hirshfield. “If you don’t think at all, you think of them as opposites,” she said. “They are allies in discovery.”

In one far corner of the subterranean network sat tables for making protest signs, with markers, construction paper and glue-sticks. For the first few hours of the event, though, people were more interested in the bar.

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