NEW YORK (AP) -- What sort of staying power does it take for a protest movement to be judged a success?
This year, without a centralized team of senior leaders, perhaps the largest protest movement in U.S. history has been unfolding nationwide since the May 25 death of George Floyd at the hands of Minneapolis police. By some calculations, more than 15 million Americans have taken part -- decrying racial injustice, reinforcing the message of Black Lives Matter.
There's no way to know now what the movement's legacy will be -- whether it will wither or compel major breakthroughs in curbing racism and inequality. But at this moment, other major protest movements of the past can offer clues about what endures or what, at least, leaves a tangible legacy.
"It's important to see the changes over time and not be discouraged," says Beth Robinson, a history professor at Texas A&M-Corpus Christi.
By some measures, it took the U.S. women's suffrage movement more than 70 years before it won voting rights for women. In the late 1980s, HIV/AIDS activists motivated by anger and fear made huge advances in just a few years thanks to confrontational protests.
The U.S. civil rights movement of the 1950s and 1960s achieved monumental changes over 15 years, including landmark federal laws. Yet racism and discrimination remain pervasive problems today.
"After Martin Luther King was assassinated, the movement kind of fractured and lost momentum," says Tyler Parry, a professor of African American history at the University of Nevada-Las Vegas.
The civil rights movement had some fundamental assets that helped sustain it, according to James Ralph, a Middlebury College historian. It had multiple prominent leaders in addition to King, and multiple national organizations that generally agreed on key goals even as they sometimes differed on tactics. That approach produced such tangible successes as the Civil Rights Act of 1964 and the Voting Rights Act of 1965.
This year's protest movement has mobilized more people and is more diverse. But it's too early to gauge what results it will achieve. Parry advises the new wave of activists to maintain the multiracial nature of the movement and work doggedly at every level to address inequities.
"What the modern movement needs to do is not be complacent if one or two things change," Parry says.
Protest movements have been at the core of U.S. history since before independence. The American Revolution itself commenced after years of protests against British-imposed taxes.
The war had barely ended when, in 1791, the Whiskey Rebellion flared -- a multistate protest against a liquor tax imposed by the new federal government. Anti-slavery protests hastened the Civil War's outbreak. The Seneca Falls convention in 1848 is widely considered the launch of the women's suffrage movement, yet women didn't gain voting rights until 1920.
Compared to that long struggle, the protests of HIV/AIDS activists achieved tangible goals within a few years of organizing in the 1980s. Activists staged "die-in" demonstrations, provoked mass arrests, and in 1988 converged by the hundreds outside the Food and Drug Administration's headquarters for day-long civil disobedience.
In response, the FDA agreed to speed testing and approval of new therapies -- a key step in curbing the high death toll from AIDS. Activist Larry Kramer, who died in May, said the protesters' sense of rage made a difference.
"Until you have anger and fear, you don't have any kind of an activist movement," he told Metro Weekly, a Washington-based LGBT publication, in 2011.
The largest single-day protest in U.S history -- the Women's March -- came on Jan. 21, 2017, the day after Donald Trump's inauguration. An estimated half million people marched in Washington, supporting women's rights and assailing Trump's misogynistic remarks. Millions more marched in other U.S. cities.
Assessing the march's impact is difficult. With Trump in office and Republicans controlling the Senate, there's been no breakthrough legislation on reproductive rights, immigration or other issues. Yet the mobilization lent strength to the MeToo movement, which began nine months later.
Some protest movements are short-lived but leave enduring legacies. Consider the Occupy Wall Street movement in New York City in 2011. It was criticized for lacking racial diversity and a specific agenda yet helped change the discourse about economic inequality with its "We are the 99%" slogan and denunciations of the wealthy 1%.
Nelini Stamp, a director of strategy and partnerships for the Working Families Party, cites Sens. Elizabeth Warren and Bernie Sanders as politicians whose economic platforms reflect the spirit of the New York protest.
In 2006, millions turned out to protest legislation in Congress seeking to classify undocumented immigrants as felons. The bill passed the U.S. House but died in the Senate.
Chris Zepeda-Millan, a professor of Chicana/o studies and public policy at UCLA, credits the protests for stopping the bill and encouraging Latino voter registration. But he said the protests also intensified congressional polarization, dimming prospects for any immigration overhaul and citizenship for undocumented immigrants.
Congress also failed to pass tough new gun-control measures in the aftermath of the massive March for Our Lives protests organized in 2018 by students from the Parkland, Florida, high school where a gunman killed 17 people. Nonetheless, gun-control activists have taken credit for numerous election victories, notably helping Democrats take control of Virginia's legislature in 2019.
One advantage for U.S. protest movements: Government security forces generally permit them to mobilize. The recent deployment of federal tactical teams in Portland, Oregon, outraged protesters and Oregon officials but has been the exception, not the norm -- a contrast to nations where protest movements have violently repressed.
Texas A&M's Robinson emphasizes that protest movements produced many of the freedoms and protections Americans treasure, including several Depression-era initiatives undertaken during the New Deal. Yet, she says, those reforms didn't fully benefit women or people of color, setting the stage for the new wave of dissent from the 1950s through the 1970s.
"With protest movements, it's three steps forward, two steps back," Robinson says. "It's always going to be a long march to justice."
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