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America is so divided in 2016 that one half of the electorate can barely understand the other. Comprehending the Donald Trump phenomenon has become the dominant theme of US election coverage, crowding out the usual questions a presidential campaign might raise about economic and foreign policy.

The question is not about the candidate himself. Trump hasn't hidden who he is - he hasn't concealed his attitudes to women and racial minorities behind sly innuendo, and his views on how he will deal with global problems are out there for all to see.

What has puzzled people who won't be voting for him, is how tens of millions of their fellow citizens could still be willing to support him. Who are they?

The answer is to be found in understanding what I call "The Bloc".

The Bloc, roughly 40% of the electorate, doesn't represent a majority in the US but it is the country's largest unified group of voters. It has lost four of the last six presidential elections but has mostly remained in control of one or both houses of Congress, and many state legislatures, making it almost impossible for Democratic administrations to govern the country.

The Rust Belt is the Dust Bowl of our times Todd Gitlin, Columbia University

The Bloc wasn't created by the Great Recession of 2008 or by the attacks of 9/11 or the bursting of the Dotcom Bubble in 2000. It wasn't even created by the economic bogeyman of this election, Nafta - the 1994 North American Free Trade Agreement.

Instead The Bloc was created by a series of events that, in an almost geological process, added sedimentary layer after layer of voters to the Republican Party.

A new layer is laid down whenever there is "another round of economic displacement", says Todd Gitlin, professor of sociology and journalism at Columbia University, the new voters being overwhelmingly whites who had previously been Democrats.

Image copyright Walker Evans/Getty Images Image caption Does Trump's appeal have its roots in the depression of the 1930s?

Gitlin traces its origins to the Dust Bowl of the 1930s, when many farmers and their families were driven from Oklahoma and Kansas by drought. Most ended up in southern California and their children were among the early supporters of the ultra-right-wing John Birch Society and the 1964 Republican presidential candidate, Barry Goldwater.

In the late 1960s many southern whites also joined The Bloc following the enactment of the Civil Rights Act and Voting Rights Act, which finally delivered - 100 years after the end of slavery - on the promise of equality for African Americans.

Then in the late 70s and early 80s, as factories in the North-east and along the Great Lakes shut down, communities like Johnstown in western Pennsylvania were devastated. Steelworkers in the town, even with their strong trade unions, were thrown out of work. People were forced to leave the region in vast numbers to find jobs elsewhere. Those jobs are nowhere near as secure as the factory jobs that sustained generations and allowed stable communities to grow. It was a displacement every bit as wrenching as that of the 1930s.

"The Rust Belt is the Dust Bowl of our times," says Gitlin.

Image copyright Getty Images Image caption Inside the Rust Belt: A closed factory lot in Warren, Ohio

Others, for whom the Supreme Court's 1973 decision in Roe v Wade (which gave women abortion rights) became the single issue on which they vote, joined The Bloc as well.

I was passed over a few times for promotion for a really good job because I wasn't a minority - that's a flame that can be fanned into hatred Bonnie Cordova, Retired teacher

But both of America's major parties are grand coalitions.

Over the last 50 years, while disaffected Democrats were making common cause with the traditionally wealthy Republicans, a new Democratic Party was taking shape as well.

Its make-up is diverse, its policies dedicated to using civil rights law to undo centuries of job discrimination against minorities and women through preferential hiring, as well as pushing for socially liberal legislation on sexuality.

An overwhelming majority of African-Americans, and significant majorities of Hispanics and Asians vote Democrat, while university-educated whites dominate the party and give it its worldview and voice.

Image copyright Getty Images Image caption Democrats draw on their support from a diverse and metropolitan spectrum of supporters

Identity and gender politics are a regular feature of life and discussion on university campuses and in the mainstream news media. Much of the debate, from the Democratic side this year, has been about white male identity. The New York Times has run dozens of articles about "angry white men".

A recent article was titled Men Need Help. Is Hillary Clinton the Answer? It was written by the paper's recently appointed gender editor, Susan Chira. Interestingly, in the late 1960s and 70s, when The Bloc began its rapid growth, it had a full-time labour correspondent. It doesn't any more.

This difference in thinking about the world provides a glimpse into America's terrible division.

When Donald Trump, speaking in his unique way, attacks Nafta for taking away American jobs, The Bloc cheers. When he attacks Democrats and their preference for gender and identity politics with its "politically correct" speech codes, the men and women of The Bloc nod in agreement.

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Most Democrats shake their heads in disbelief and simply can't understand how anyone could vote for the Republican candidate.

One Clinton supporter I met while making my radio documentary, The Unswayables, did understand. Bonnie Cordova, a retired schoolteacher, and I were both watching the second presidential debate at the Bohemian Beer Garden in the New York borough of Queens. I asked her afterwards if she understood why people might vote for Trump.

She did. "I taught in inner city schools for 30 years," Cordova explained. "I was passed over a few times for promotion for a really good job because I wasn't a minority and I was working at a school where the kids got free dental and optical treatment because they were immigrant children and I was having trouble affording it for my kids." She acknowledged that made her resentful. "That's a flame that can be fanned into hatred. You have to rise above it."

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Not everyone can rise above it.

And so American society and government is split. Dangerously.

But The Bloc isn't permanently moored to the Republican Party.

The Democrats would find governing easier if they brought some of its voters back. The best way to do that is directly related to employment. Economic displacement created The Bloc. Stable employment might whittle it away.

But Todd Gitlin, who has been a leader of left-liberal political movements in the US since the 1960s, doesn't hold out hope of this happening soon.

"What's the social democratic vision? There are fragmented visions, there are sectoral visions, little visions. There are no passionate centres. There is no luminous idea of what the world would look like if it were better," he says.

"People don't have a conception of the work of being a citizen and actually governing yourselves as a people. It's not just a problem on the right. It's that we don't have on the left a sustainable idea about political work."

The Bloc has been formed over a period of at least half a century. It will take more than one presidential election - dominated by the most unpresidential candidate in American history - to chip away at it.

It's not clear who on the Democratic side has the patience to give up the years that will be necessary to persuade these voters if not back to the Democratic fold at least to a position where compromise - and functioning government - is possible.

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