Image copyright Getty Images Image caption Trump has tapped into anger in the rust belt

Only an angry time could produce Donald Trump as a presidential candidate.

Talking, on the eve of the convention in Cleveland, to a Republican with decades of political campaigning under his belt, we agreed that no one in modern times has broken the rules like Trump.

With his style, his ego, his language and his willingness - indeed, his determination - to fly solo.

But the audacious outsider has found his moment. In America, there are angry people enough to want someone who doesn't just disagree with his opponent but calls her "crooked Hillary" and suggests she should be in jail, and fashions speeches that his conservative opponent in the primaries, Senator Ted Cruz, called "vulgar and coarse".

He is the candidate for whom anything goes. And you don't have to go far from Cleveland to find the reason why he can do it.

Just south of here you reach the American Rust Belt, towns with empty steel mills, ghostly factories and worked-out mines. Once the industrial core of the country, a cauldron of work and prosperity, this territory is now the emblem of decline.

Youngstown, Ohio, catches the flavour. Four decades ago, its blast furnaces lit up the sky.

Image copyright Getty Images Image caption Cleveland has declined since the 1950s

With Pittsburgh further south it was the heart of the steel industry, the iron ore coming down from Lake Erie and the long trains hauling the steel east and west across the country.

But the population has more than halved, the mills are mostly gone, and the people are angry.

I met third generation steel workers whose immigrant forebears had come to the mid-West with the certain promise of work and a better life.

The young generation now gets out while it can. So the city is angry - about jobs, the manufacturers who've moved abroad, about falling incomes, about immigration. This is Democratic Party territory, but it is made for Trump.

Mark Munroe, the Republican chairman here, told me he nearly fell off his chair on primary election night in the spring when he saw how many votes were piling up for his candidates, and especially Trump.

Jose Arroyo, a local official for the United Steelworkers union and a Democrat stalwart, told me the same thing.

"We had a wake-up call. Now we need to educate our people," he said.

Educate? The trouble for Democrats is that Trump has produced the politics of the gut.

How would he deal with Vladimir Putin? "Just watch me!" The Chinese, with whom he wants a trade war? "Just watch me!" Iran? You've guessed it.

He distils it into two claims, which will dominate the convention here - America safe again, and American great again.

And when someone asks how, he says he is a deal-maker. All you need to know.

This style has stirred up formidable support. Among people who have felt for two or three decades that the promise of America has faded, and that perhaps its greatest days are behind it, not ahead, that message is enough.

Image copyright John Moore Image caption Ohio is symptomatic of the economic troubles seen by the American rust belt

But don't doubt the undercurrent of discomfort, even alarm, among Republican stalwarts.

Even some of them who've endorsed him - maybe with the lukewarm enthusiasm of Paul Ryan, the House Speaker, who's chairing the convention - live with the fear that this is a candidate who will fail spectacularly, and perhaps take the majorities in the Senate and the House with him.

Few will say that this week, but many will suspect that it's going to happen.

And yet…

That anger, welling up in communities where jobs have gone, and doubtless stoked-up by the horrors of the street shootings in recent days, is a potent force.

Trump, the lightning rod, is aglow with its power. He feels it, and he feeds it back. But will it be enough?

Image copyright Getty Images Image caption Donald Trump's message has struck a chord in Ohio

The closer we get to election day the more people will think of him not as a candidate - a repository for their fears and anger - but as a potential president.

And, even if they are part of the huge number of Americans who are still reluctant to embrace Hillary Clinton, they may draw back. Trump has found that dividing people is easy. Pulling them together, especially when anger is your business, is more difficult.

The Trump-Clinton polls are close, and moving up and down in the swing states, so it is too early to take the temperature of the real campaign. But we'll learn more about Trump this week.

Has he only one voice? Because if he has, it may not be enough to see him through.

James Naughtie is BBC News Book Editor and presents Bookclub on BBC Radio 4. He was a presenter on the Today programme from 1994 to 2015.