As the Occupy Wall Street movement spreads and gains attention, it’s tempting for many people to dismiss it as a bunch of lefty kooks in much the same way people tried to dismiss the tea party as, well, a bunch of right-wing kooks.

Tempting, but mistaken. Listen to Doug McAdam, a professor of sociology at Stanford University who studies populist movements like these. He places the current uprisings represented by the tea party and Occupy Wall Street alongside the unrest in the Great Depression and the activism of the 19th-century populist movement as the most significant economic protests in U.S. history. And he makes a strong argument to back that up.

We’re accustomed to thinking that the era of American radicalism is something from a bygone era. McAdam noted that the most recent economic uprising of comparable scale occurred in the early days of the Great Depression, which was marked by widespread protests and violence.

But, he pointed out, “This is a very tumultuous time in the U.S., politically, economically and socially.”

With both Occupy and the tea party, people have underestimated them because they seem messy, chaotic, muddled. But McAdam says such incoherence is typical at the start of protest movements.

“This is the way movements always develop,” McAdam said. “We look back at them as unitary things. But really, the civil rights movement was a collection of local struggles.”

The most relevant ancestor of today’s uprisings is the original populist movement of the 1870s. These agrarian populists were fueled by anger that banks, large corporations and the government were in cahoots to deprive farmers of their land, capital and livelihood. Sound familiar?

The populist revolt became the largest grass-roots democratic movement in this nation’s history as farmers sought to seize control of their economic destiny. As an undergraduate at Duke University in the late 1980s, one of my favorite history classes was “The Insurgent South,” taught by professor Lawrence Goodwyn. One of Goodwyn’s most important books, “The Populist Moment,” examined the dynamics of the agrarian uprising.

“A large number of people in the United States discovered that the economic premises of their society were working against them,” Goodwyn wrote in the introduction. “These premises were reputed to be democratic — America after all was a democratic society in the eyes of most of its own citizens and in the eyes of the world — but farmers by the millions found that this claim was not supported by the events governing their lives.”

Those complaints would later be echoed during the Great Depression, and we hear them again today.

But just because neither the tea party nor the Occupy movement can articulate a coherent solution to the problem does not mean we can brush aside their critiques about the fundamental decay of our economy and politics, and their frustration over feeling powerless to change it.

“We’ve simultaneously got a conservative populist movement and a progressive populist movement happening at the same time,” said Rory McVeigh, director of the Center for the Study of Social Movements at the University of Notre Dame. “There’s a sense on both sides that it’s us against that unnamed force out there running the world. That there’s a cultural elite who doesn’t understand the average person.”

The two movements, of course, point the finger at different villains.

For the tea party, it is the government, and the solution is to paralyze it. For the Occupy movement, it’s the wealthy and the banks as proxies for big business, with the remedy being some undefined desire for greater accountability and a fix to the country’s growing economic inequality.

There’s another lesson for these two movements from history. The populist movement eventually stalled when it stepped into national electoral politics, becoming the Populist Party and nominating William Jennings Bryan as its presidential candidate in 1896. We’ve already seen the tea party move into politics, and it seems inevitable that the Occupy movement will be tempted to do so as well. And in a way, that’s a shame, because their larger message, like that of the populists, may get lost in the political arena.

Still, the original populist movement’s ideas about economic justice would be embraced decades later by Woodrow Wilson and FDR. The real issue will be whether the tea party or Occupy movements can take the next step toward articulating principles and ideas that could reframe the way people view their relationship to government and big business.

That will be the measure of whether either movement succeeds in seizing this historic moment to change the course of the country for decades to come.

Contact Chris O’Brien at 415-298-0207 or cobrien@mercurynews.com. Follow him on Twitter at sjcobrien and read his blog posts at www.siliconbeat.com.