President Donald Trump's behavior after impeachment has alarmed experts on authoritarianism and fascism and led to calls for mass protests.

Experts on civil resistance say it would take millions flooding the streets of major US cities to compel Republicans to change their behavior and take a stand against Trump.

"Defections don't happen on their own," Erica Chenoweth, a political scientist at Harvard, told Insider. "People don't just spontaneously do it — they need to be called to do it, and mass, nonviolent protests is a way to call them to do it."

"If we really want mass protests, it can't just be in Washington, DC," Dana Fisher, a sociologist who researches activism, said. "That's the kind of disruption I think is necessary to make everybody pause."

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As the vast majority of Republicans in Congress look on with indifference to President Donald Trump's authoritarian-like efforts to tighten his grip on the US government since his impeachment acquittal, there have been increasing calls for large-scale protests in major cities across America.

"The system is enabling Trump," Jason Stanley, a Yale philosophy professor who wrote "How Fascism Works," recently told Insider. "There need to be mass protests."

Republican leaders have stood quietly by as Trump purges officials who testified in the impeachment inquiry or are suspected of some kind of disloyalty. They barely flinched as Trump called for a lighter sentencing recommendation for his longtime informal adviser Roger Stone, which was promptly delivered on the orders of his attorney general. And they're not raising major objections as Trump brings in an inexperienced loyalist to be America's top spy chief amid warnings from the intelligence community of ongoing Russian threats to the 2020 election.

For those troubled by these unprecedented moves, what would it take to successfully resist Trump?

There would need to be "large-scale disruption," Dana Fisher, a sociologist at the University of Maryland who studies protest movements, told Insider. Fisher pointed to the example of the nonviolent tactics employed during the US civil-rights movement and the recent protests in Paris that have been mostly peaceful but highly disruptive.

"That's the kind of disruption I think is necessary to make everybody pause," Fisher added. "The kind of pause that's needed for Republicans to reconsider how we may very well be ... approaching authoritarianism like Stanley and others have talked about."

Firefighters in Paris simulated setting themselves on fire during a January 28 demonstration to protest against working conditions. Charles Platiau/Reuters

Protesting works

There have been numerous mass protests in the Trump era. The 2017 Women's March, which took place the day after Trump was inaugurated, marked the largest single-day protest in US history. It's estimated at least 4.2 million people participated in the march in various events across the country.

Observers often say "protest doesn't work," but that view isn't supported by the Women's March and other early demonstrations against Trump.

Fisher's book "American Resistance" documents a direct connection between protests against Trump in his first two years and the so-called blue wave in the 2018 midterms that saw Democrats take back the House while electing a historic number of women and minorities. She surveyed people at these protests and tracked what they did after.

"The protests became the beginning of activism for all these Americans across the country, who then went back into their communities and congressional districts, mobilized and worked around the election in 2018 to make the blue wave happen," Fisher told Insider.

Protesters during the Women's March on Washington on January 21, 2017. Mario Tama/Getty Images

Erica Chenoweth, a political scientist at Harvard who's done extensive research on the power of nonviolent resistance, also emphasized the link between the early anti-Trump demonstrations and the 2018 midterms, as well as other trends.

"We've already seen at least some important electoral impacts of even that single day of protests, and then if you take into account the many other acts of protests that have happened in the US beyond the Women's March, there definitely has been an important impact on agenda setting, the progressive shift in the Democratic party, and a number of other preventative outcomes," Chenoweth told Insider.

In her research, Chenoweth looked at all major violent and nonviolent campaigns for the overthrow of a government or a territorial liberation from 1900 to 2006 and found that nonviolent civil resistance was twice as successful as violent campaigns. In short, there's a lot of empirical evidence that large-scale, nonviolent protests work. Numerous world leaders, from Iceland to South Korea, have stepped down or been pushed out in recent years after mass protests.

'If we really want mass protests, it can't just be in Washington, DC'

There have been ongoing demonstrations against Trump and his agenda since the 2018 midterms, but they have seemingly done little to change the president's behavior, which has become increasingly alarming to experts on authoritarianism and democracy.

And though some Republicans have expressed concern and criticized Trump, it has not been enough to significantly alter his approach to the presidency. Only one Republican, Sen. Mitt Romney of Utah, voted to convict Trump in his impeachment trial. Even as several GOP senators condemned the president's actions, which involved urging Ukraine to investigate his political rivals as he seeks reelection, no other Republicans joined Romney.

Fisher said the GOP's undying loyalty to Trump was unlikely to change unless the protests against the president became larger and more confrontational.

"We've seen a lot of protests in the past few years since the inauguration, starting with the Women's March 2017 ... but it's been extremely peaceful and nonconfrontational," Fisher said. "What that means is that the organizers are all following the law. When we think about the type of protest needed to respond to authoritarianism ... we're talking about confrontational protest that's disruptive."

Confrontational or disruptive protests do not mean blowing things up, setting fires, or throwing stones at the police, Fisher said, but they do mean blocking traffic and stopping business as usual. "That's what confrontation is all about," Fisher said, adding that these protests would need to take place in every major city in the US to be effective.

"If we really want mass protests, it can't just be in Washington, DC," Fisher said.

'Defections don't happen on their own'

Having mass numbers involved in protests is important in terms of their likelihood of success, but not the only important factor.

Chenoweth's research found that if at least 3.5% of the population participated in nonviolent protests in an active and sustained way, it always succeeded in fostering political change. None of the campaigns she looked at over the course of roughly a century failed when they achieved the 3.5% threshold, and every campaign that surpassed it was nonviolent.

Based on the 3.5% rule, roughly 11 million Americans would need to actively participate in a campaign against Trump for him to be compelled to accept some of the movement's demands or step down. But Chenoweth cautioned that it's also not as simple as getting 3.5% of the population into the streets for a movement to be successful.

"The 3.5% rule is useful in understanding the degree to which people have actually built such popular, broad-based legitimacy that they're able to pull that number of people out," Chenoweth said.

Chenoweth said that for change to really happen there needed to be defections.

"These are people who are either security forces, police, economic or business elites, or people that are political opposition leaders or political loyalists, who suddenly decide they're not supporting the status quo or they're not supporting the incumbent anymore," Chenoweth said. "They don't necessarily come over to the side of the movements in every case, but even stopping their overt support for the opponent can be a huge boost for a campaign."

With Trump, the defections so far have been too small or too short-lived to matter, and since his acquittal he's purged anyone viewed as disloyal from the government.

"Movements don't just win because they have a mass uprising, but also that multiyear strategy where it's building capacity to create defections, either through persuasion or imposing direct costs, nonviolently, on loyalists," Chenoweth said. "It's just not that everybody is suddenly like, 'Wow, there's so many people in the streets, we really believe what they stand for now!'"

"Defections don't happen on their own. People don't just spontaneously do it — they need to be called to do it, and mass, nonviolent protests is a way to call them to do it," Chenoweth added. "Part of the main function of nonviolent resistance is to signal ... to regime functionaries or people who are loyal to the regime that they don't have to be."

'People still believe in America'

A large part of the reason there have not been protests on the scale that would be needed to inspire substantive change within the Trump administration and among Republicans is that Americans still have faith in the electoral process.

And this is in spite of the fact Trump was impeached over soliciting foreign interference in the 2020 election in a complex scheme that involved freezing nearly $400 million in congressionally approved security assistance.

President Donald Trump with Ukrainian President Volodymyr Zelensky at the InterContinental Barclay New York hotel during the United Nations General Assembly on September 25. AP Photo/Evan Vucci

Trump was impeached by House Democrats who accused him of abusing the powers of his office by attempting to extort Ukrainian President Volodymyr Zelensky into publicly announcing an investigation into former Vice President Joe Biden, a top-tier 2020 Democratic candidate, to drag Biden's name through the dirt over baseless allegations. Trump also wanted Zelensky to launch an inquiry into a bogus conspiracy theory that Ukraine, not Russia, interfered in the 2016 US election.

Fisher said that if Trump were reelected, that's when the US could really see millions pour into the streets in a more aggressive, disruptive way. But right now, the various organizations and movements she's studied and communicated with are urging people against such tactics and toward mobilizing around the 2020 election. "My interpretation of their support for such peaceful, tame, and what some might even call bland protests is because they're really focused on the election," Fisher said.

"This level of tamping down of the outrage that we have seen burbling up over and over again during this administration, it's only possible because people still believe in America, or America as a promise of democracy," Fisher said, adding that disruptive protests could be divisive as these groups focused on the election.

"The day after the election is when it could happen," Fisher added. "And it could happen because Trump gets reelected, it could happen because Trump is unwilling to leave the White House ... There are a number of different trajectories that get us to a real crisis of American democracy, but we're not there yet."

"We're right on the precipice, right at the edge," she said.

"Nonviolent resistance is not the only game in town, but it is also not something to underestimate in terms of its ability to help us maintain and improve the democratic experiment," Chenoweth said.