GEOFF BENNETT: Two years ago, Judy Woodruff began traveling the country to examine the roots of division for her series, America at a Crossroads.

Tonight, she returns with a conversation with someone who has spent his life considering such questions.

ROBERT PUTNAM, Harvard University: Fundamentally undermining the American constitutional order, changing us from being a democracy.

JUDY WOODRUFF: Now 84 years old, Harvard Professor Emeritus Robert Putnam has spent decades studying how American society evolved from one that, however flawed, was steadily moving towards greater connection, equality, cooperation and cohesion... JOHN F. KENNEDY, President of the United States: Ask not what your country can do for you.

Ask what you can do for your country.

JUDY WOODRUFF: ... to one that in more recent decades has been defined by growing isolation, distrust, inequality, and political discord.

DONALD TRUMP, President of the United States: My recent election is a mandate to completely and totally reverse a horrible betrayal and all of these many betrayals that have taken place.

JUDY WOODRUFF: To the point that Putnam now worries these forces threaten to upend the Constitution.

ROBERT PUTNAM: They're talking about not obeying court orders.

I mean, come on.

That's -- the court system, better or worse, is currently the last bulwark of our democracy.

So we're awfully close to breaking the bounds that have kept us -- our democracy safe.

How many in the room are on a bowling team, bowling league?

JUDY WOODRUFF: Paul Solman first profile Robert Putnam on this program 30 years ago, when Putnam published his essay "Bowling Alone," which would become his groundbreaking book showing that, since the high water mark of the 1960s, Americans had become steadily more isolated and that this was weakening civic engagement and undermining our democracy.

ROBERT PUTNAM: That is a primary cause of the Trump phenomenon.

That's true.

You can see it in the data, but you don't have to trust me.

Steve Bannon has said publicly, back in the day, when they were trying to figure out how they could get Trump elected, they read this book by this crazy academic called "Bowling Alone," and that guided their -- I'm not proud of this, but that guided their strategy, because they thought, just as I had been writing, that, when people are socially isolated, as we are increasingly, they become vulnerable to populist appeals.

So that's the first point.

We are increasingly socially isolated.

And that makes our country vulnerable to, I was going to say fascism.That isn't quite true, but it's close to being true.

The poor kids who live here now are living in a completely different universe for the rest of the kids in town.

JUDY WOODRUFF: In 2015, Putnam chronicled another major concern with his book "Our Kids," the growing gap between increasingly well-off college-educated Americans and everyone else, now one of the greatest predictors of who supports Donald Trump.

ROBERT PUTNAM: Well, until we fix the underlying problem -- basically, there's two pairs of underlying problems, increasing social isolation and especially in the non-college-educated part of our population -- we're constantly vulnerable to that same kind of pressure for the same reason that we were vulnerable when he came along.

JUDY WOODRUFF: I met Putnam last week just off Washington Square Park in Manhattan at Judson Memorial Church, founded in 1890 by American Baptist Minister Edward Judson with money donated by John D. Rockefeller, with a mission to provide aid and comfort to the poor immigrants living in the squalid tenements nearby.

WOMAN: You come and have a place that is beautiful to look at away from what they were experiencing in the tenement houses.

JUDY WOODRUFF: Today, Putnam sees strong parallels between that era, the post-Civil War period known as the Gilded Age, and our own.

ROBERT PUTNAM: America was extremely polarized.

Politics then was very tribal, just like it is now.

Inequality was very high then.

That was the last time the gap between rich and poor was anywhere near as large as it is now, so very polarized, very unequal, very socially disconnected, very socially isolated, because then the industrialization meant that millions of people had just moved from a village, a village in Sicily or a village in Iowa, to the big city.

They had left their family and friends and connections behind.

So, they were very disconnected, just like we're very disconnected.

And they were very self-centered, if I can put it that way.

Americans in that period were focused on I and not on the we.

JUDY WOODRUFF: Putnam's most recent book, "The Upswing," describes these many parallels in detail and how out of those challenges came an explosion of new civic, religious and social groups, the Boy Scouts, the NAACP, the Rotary Club, the General Federation of Women's Clubs, as well as the many reforms that came to be known as the progressive era, the federal income tax, women's suffrage, labor rights and more, changing the course of the following decades.

ROBERT PUTNAM: All of a sudden, we began to become more equal, less polarized, more connected, and a greater sense that we're all in this together.

JUDY WOODRUFF: What did they do?

ROBERT PUTNAM: Well, there are both positive and negative lessons, actually, I have to say.

And I thought, for sure, I knew what would change first.

I thought it was the economics, and I thought maybe the economics would change first.

We'd begin to become more equal economically, and then our politics would improve and so on.

The one thing the data show is, that's not true.

Economics was the last thing to change.

So then what was the first thing to change?

And, to my shock, it was cultural change.

It was a moral revival is the way I want to put it.

People began to say, wait a minute, it's not all about us.

We have obligations to other people.

JUDY WOODRUFF: One example he points to was the Triangle Shirtwaist Factory fire just a few blocks from where we were speaking.

In 1911, a fire broke out there in a sweatshop, trapping garment workers locked inside.

Dozens jumped to their deaths; 146 women died in all, shocking the conscience of the city and beyond, including a young woman named Frances Perkins, who was having tea with friends nearby.

ROBERT PUTNAM: Up to that point, she had been thinking of herself as a debutante.

And she said, that's evil.

We have obligations to those people.

Almost immediately, she became a social reformer right here in New York City, and eventually she became FDR's secretary of labor, the first woman Cabinet member in American history.

And it was all because of that moral moment in which she realized she had obligations to other people.

And that's one example among what was happening a lot.

I sort of think that it's going to be hard for us to turn things around in America until we begin to recognize we have obligations to other people that are at least as important as ours.

NARRATOR: This is a film about why you should join the club and why the fate of America depends on it.

JUDY WOODRUFF: Putnam's life story and work have been recently captured in the documentary "Join or Die," encouraging people across the country, especially young people, to again seek out organizations and connection in their own communities, to find issues they're passionate about, and to effect change from the bottom up, like the progressives did, but to also go beyond that movement to include Americans of all colors, backgrounds and beliefs.

One of your points is that this is something that happened at the grassroots level.

JUDY WOODRUFF: It didn't come out of Washington.

It didn't come from the White House, handed down.

ROBERT PUTNAM: Or Harvard or whatever, yes.

JUDY WOODRUFF: Do you believe that the ingredients are there for that to happen again?

ROBERT PUTNAM: It is happening now, I mean, not enough of it.

We need to -- well, your show, this whole series is doing that.

You and I both know from our own personal experience that this is happening, and a lot more of it is needed.

But that's where it'll begin.

I am not a determinist.

I don't think any of these things are guaranteed to happen.

I don't think it was guaranteed to happen last time.

I don't think there was some big cycle in the skies or God or something that was saying, oh, there will be a progressive era here.

I think it happened because a smallish number of young people around 19 -- between 1900 and 1910 decided, like Frances Perkins right here, she decided then she was going to help change America.

It was not inevitable.

It is not inevitable this time.

But it could happen.

That's why I'm saying the distinction between will it happen.

I don't know whether it will happen, but I know it could happen, because it did happen.

JUDY WOODRUFF: And that will be the focus of many of our stories this year.

For the "PBS News Hour," I'm Judy Woodruff in New York.