GEOFF BENNETT: The Smithsonian Institution established by Congress in 1846 is the world's largest museum, education and research complex.

It's a public-private trust that has long operated at arm's length from the White House, but the Smithsonian now finds itself under unprecedented scrutiny from the Trump administration.

Our senior arts correspondent, Jeffrey Brown, reports now on how a group of citizen historians are responding to the administration's moves, part of our series Art in Action, exploring the intersection of art and democracy and our Canvas arts coverage.

JEFFREY BROWN: On a recent weekend, Virginia resident Barbara Michelman went to her favorite Smithsonian museum in nearby Washington, D.C., the Hirshhorn, renowned for its collection and exhibitions of contemporary art.

BARBARA MICHELMAN, Volunteer, Citizen Historians for the Smithsonian: The art there isn't like anything I ever saw in books or magazines in school growing up.

JEFFREY BROWN: But this was no ordinary visit.

Instead of viewing the art, this time, she was there to document it.

BARBARA MICHELMAN: When I saw the request for volunteers, I thought art is one of the first things to be attacked.

And this seemed like a direct action for something that I feel is worth protecting, which is freedom of artistic expression.

JEFFREY BROWN: Armed with her cell phone camera, Michelman carried out her assignment, photographing all art and signage in one section of the Hirshhorn, including works by Jackson Pollock and Janet Sobel, another pioneering abstract expressionist.

It's part of a new grassroots effort to document everything on display at the Smithsonian's 21 museums and National Zoo.

CHANDRA MANNING, Citizen Historians for the Smithsonian: We call it Citizen Historians for the Smithsonian, because we think it captures the collective and communal effort that the project is.

JEFFREY BROWN: The project is the brainchild of Chandra Manning and James Millward, two Georgetown University history professors undertaking it as private citizens, independent of the university.

JAMES MILLWARD, Citizen Historians for the Smithsonian: The museum itself has excellent documentation, archives all of their data.

They have beautiful images of everything that we have taken amateur images of.

But the point is not to try to duplicate what they're doing, but to have our own copy out among the people that we could refer to if necessary in the future.

JEFFREY BROWN: Last month, the White House announced a sweeping review of the Smithsonian Institution, including current and future exhibitions, wall texts, and even some internal communications.

Within 120 days, the White House said, museums will be required to begin so-called content corrections -- quote -- "replacing divisive or ideological-driven language with unifying, historically accurate and constructive descriptions."

DONALD TRUMP, President of the United States: We want the museums to treat our country fairly.

We want the museums to talk about the history of our country in a fair manner, not in a woke manner or in a racist manner, which is what many of them, not all of them, but many of them are doing.

JEFFREY BROWN: It's part of a broader attempt by the president in his second term to reframe cultural and historical narratives.

Millward, an expert on Chinese history, said the review spurred him to action.

JAMES MILLWARD: One of the things about history in China is that it's very, very often, for thousands of years, been controlled by the state.

And I really don't want to see our own history be Tiananmenized the way that the Chinese Communist Party tries to censor its own past.

JEFFREY BROWN: He approached Manning, who as a graduate student had worked as a National Park Service ranger.

She suggested mounting a campaign similar to one organized in response to an administration review launched in May that aimed to remove signs in national parks deemed to have improper partisan ideology.

The campaign called Save Our Signs and founded by a group at the University of Minnesota asked park visitors to take and submit photos of signs to a public Web site to capture any that might disappear.

CHANDRA MANNING: I am a historian of the United States who has spent my career trying to understand the full story of our country.

And it really concerns me to simplify and even falsify that story into a false uniformity because, if you can erase people from the past, it becomes legitimation for erasing people from the present and the future.

JEFFREY BROWN: While the national park review is ongoing, administration officials have ordered the removal of signage and materials related to Native Americans and slavery, reportedly including a reproduction at Georgia's Fort Pulaski National Monument of the Scourged Back, a well-known Civil War era photo of a formerly enslaved man with scars on his back.

Manning says the photo was circulated widely at the time.

CHANDRA MANNING: Americans in the 1860s could handle the hard truths about the past and could be inspired about them to do better.

I think that it's striking that the administration doesn't seem to think that Americans of the 21st century have the backbone of Americans of the 19th century.

JEFFREY BROWN: Manning and Millward teamed up with graduate student Jessica Dickinson Goodman, who helped organize hundreds of volunteers and set up a system to catalog their photos and videos.

JESSICA DICKINSON GOODMAN, Citizen Historians for the Smithsonian: For example, if you wanted to look up and see what the transatlantic slave trade exhibit had looked like on a particular date, you can go in and see.

JEFFREY BROWN: While exhibition rotations are routine, the project aims to track changes brought on by this unprecedented involvement by the White House.

JAMES MILLWARD: If there are egregious attempts to censor the texts or to remove exhibits that shouldn't be removed, then we will have a record that we can quickly find and we can give the before-and-after.

JEFFREY BROWN: The historians initially reached out to neighborhood groups and local LISTSERVs for volunteers.

Word of the effort spread and they were soon inundated with offers to help.

Were you surprised by the response?

CHANDRA MANNING: Pleasantly, yes.

JAMES MILLWARD: I was overwhelmed.

I think we just passed 600 volunteers today.

JEFFREY BROWN: What was initially meant to document the eight museums highlighted in the Trump administration's review has now expanded to all the Smithsonian's museums.

In just four weeks, the project has amassed a visual record consisting of more than 25,000 photographs.

JAMES MILLWARD: It's not only about history, of course.

It's also about culture and art and about science... CHANDRA MANNING: That's right.

Yes, right.

JAMES MILLWARD: ... because the Museum of Natural History is also one of the ones on the White House list.

There, we're talking about efforts to change what we say about climate change, about the age of man, how old the Earth is, perhaps, all sorts of things deeply related to science.

JEFFREY BROWN: Retiree Katherine Pruitt started her career in the Collections Department at the Smithsonian Museum of Natural History in 1977.

KATHERINE PRUITT, Volunteer, Citizen Historians for the Smithsonian: Some of the best minds in the country are involved in curation of the collections, in design of the exhibits, and those minds have to represent a broad spectrum of scholarship, or the curator is not doing their job appropriately.

JEFFREY BROWN: After hearing about the Citizen Historian project from a former Smithsonian employee, Pruitt and her husband, Tom, volunteered, their assignment part of the National Portrait Gallery's Hall of Presidents.

KATHERINE PRUITT: We did have a chance to read how the presidents and their accomplishments had been portrayed.

It was an interesting exercise to think about.

Would any of this really be something that someone would object to as improper?

I didn't feel that way, but I did feel good about making sure that we had it documented as it exists today.

JEFFREY BROWN: The "News Hour" reached out for comment to both the White House and Smithsonian.

Neither to date has provided a response or spokesperson.

Meanwhile, this citizen action continues.

How do you define success then?

CHANDRA MANNING: For me, success will be, do Americans feel connected to this institution and like they have contributed to it?

Does the Smithsonian hear the message?

Do they hear how loved they are and how valued their work is?

JEFFREY BROWN: Manning and Millward hope to eventually make their Smithsonian collection available online to the public.

For the "PBS News Hour," I'm Jeffrey Brown.