Last Saturday, I pulled on a heavy jacket and a good pair of gloves and headed south of my hometown of Madison, Wisconsin, to meet up with some farmers. Many of them had done their chores before dawn and then brought their tractors to the edge of our state's capital city.

It was cold and windy. We could barely hear my friend Joel Greeno, a dairy farmer from the far west of the state who had put out the call to members of the Family Farm Defenders and Wisconsin Farmers Union, when he shouted: "Let's roll." Fifty tractors fired up and began the three-mile ride into Madison. Along the side of the road, there were small groups of people with signs reading: "Thank you, farmers!"

As we got closer to town, the crowds grew larger and larger. Passing cars and buses honked their horns. Drivers rolled down their windows and shouted: "Thank you! Thank you!" As our tractorcade pulled up the hill and into the great square around the Capitol, tens of thousands of Wisconsinites greeted the farmers, with chants of: "An injury to one is an injury to all" and "Solidarity!"

Joel Greeno and his fellow farmers had come to join the largest pro-labour mass mobilisation in modern American history. After Wisconsin Governor Scott Walker, a conservative Republican taking cues from corporate donors and rightwing thinktanks, moved to eliminate most collective bargaining rights for state, county and municipal employees and teachers as part of a broad plan to defund public services, Wisconsinites pushed back. The unions that were most threatened may have taken the lead. But they never stood alone.

When schools teachers in Madison walked off the job to lobby against Walker's bill, they were followed by thousands of students who marched more than two miles to the Capitol. When the teachers went back to work, parents stepped up to fill the void. The crowds at the Capitol grew from 10,000 to 30,000, to 50,000, to 70,000, to more than 100,000. Small towns across the state hosted their first ever labour rallies. And those who marched in the villages and towns of the state headed to Madison to join the rallies that grew larger and larger, and more and more diverse, as African American high school students from the urban core of Milwaukee rallied with 80-year-old farmers from towns too small to find a place on the map.

The Rev Jesse Jackson, astounded by what he was seeing, told a crowd on a bitter night: "This is a King moment. This is a Gandhi moment." And he was right. The protests in Wisconsin have captured national, even international attention, and caused union leaders in the United States, after being so battered for so long, to speak of "a rebirth of the labour movement".

But why? Is Wisconsin unique? In some ways, yes. In my new book, The "S" Word: A Short History of an American Tradition … Socialism, I recount Wisconsin's radical roots, its history of electing socialist mayors and legislators well into the 20th century and the alliances of those socialists with Robert M La Follette's progressive movement, which asked the question:

"Which shall rule – wealth or man? Which shall lead – money or intellect? Who shall fill public stations – educated and patriotic free men, or the feudal serfs of corporate wealth?"'

But many states have radical roots. Though our history has been hidden, even denied, in recent decades, America is still the land of Tom Paine and Frederick Douglass and Emma Goldman and A Philip Randolph and Martin Luther King Jr.

What I think is happening now in America is that, finally, as the corporate interests make their ultimate claim on the commons, they have created what my old friend Florence Reece described as a "which side are you on?" moment. There is no longer the easy middle ground that compromising and compromised Democratic politicians tried for so long to occupy. Now, the choices are stark: unions or no unions, public services or no public services, plutocracy or democracy?

As Wisconsinites have taken their sides, and as Ohioans and Michiganders and New Yorkers have done the same, they have found that the old divisions that so favoured the elites – white versus black, gay versus straight, native versus immigrant, urban versus rural – are abstract and meaningless. What is real is the threat of state and a nation so defined by corporate campaign contributions, corporate lobbying and corporate power that they can take away our right to organise unions, to speak in our workplaces and our communities, to petition – as the founders of the American experiment intended – for the redress of grievances.

We draw on our radical traditions, yes, but we also respond to the demands of this moment, the first moment of a new American age when peoples once divided recognise that the threats are so serious and the potential of our unity so great that we must all, in the words of the posters the farmers attached last Saturday to their tractors, "Pull together!"

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• John Nichols is appearing on panels at the Left Forum 2011 conference, 18-20 March, at Pace University, New York. Follow this week's series of articles on the theme of 'The new solidarity'