These battles are being waged as anger grows over the widening gap between rich and poor and public support for the labor movement is at a 15-year high, even as union membership continues its long, steady decline. But with nearly 15 million union members nationwide, and young people, professionals, and people of color bringing new energy to the movement, unions are showing they can still be a formidable force.

“What we’re seeing is an increasing resistance to the fundamental unfairness of a system that’s so skewed both economically and politically to the wealthy,” said Benjamin Sachs, a Harvard Law School labor professor, noting that when Uber goes public, former CEO Travis Kalanick’s stock is expected to be worth upward of $6 billion — an amount that would take a full-time Uber driver 150,000 years to make.

“Societies can’t function that are that unequal,” Sachs said.

Union have long played a major role in narrowing the income gap, according to a recent paper by economists at Princeton and elsewhere. They combed through Gallup surveys of American households going back to the 1930s and found that, when more workers were organized, the income divide was lower, improving the pay of union and nonunion employees alike.

And even when workers are unionized, they are often forced to bargain away the rights of future hires, noted Trey Kovacs, a policy analyst at the conservative Competitive Enterprise Institute. In negotiations with Stop & Shop, for instance, the United Food & Commercial Workers union accepted lower pension benefits and a delay in Sunday time-and-a-half pay for future employees. Following a six-month lockout in Massachusetts by National Grid, the United Steelworkers gave up pensions for new hires.

“You’ve created a subclass of future workers,” Kovacs said. “If they want to take this job, they’ll never have the same benefits of the people that they work alongside.”

But after several decades of unions getting “hammered” by companies during labor disputes, unions are finding a renewed zeal for their actions, said Tom Juravich, a labor studies professor at the University of Massachusetts Amherst — especially as the stock market hits record highs and average Americans realize they aren’t sharing in the wealth.

Last year, about 485,000 people were involved in work stoppages, the highest number since 1986.

The resurgence in labor activism picked up steam in the fall of 2011, when the Occupy movement brought to light the fact that a disproportionate amount of the country’s wealth belonged to the wealthiest 1 percent. But without resources or a clear-cut vision, the movement fizzled out within a few months. The following year, the Service Employees International Union launched the Fight for $15, and the ensuing protests by fast-food and other low-wage workers led to $15 minimum wages around the country, including eventually in Massachusetts.

Today, as it becomes clearer the economic recovery following the Great Recession is mainly benefiting the wealthy, unions are getting bolder about going out on strike, from teachers around the country to Marriott hotel workers in eight cities, including Boston, to the 31,000 Stop & Shop workers in New England.

With their deep pockets, organizing abilities, and political clout, unions — unlike the ragtag protesters of Occupy Wall Street — are uniquely positioned to draw attention to the widening chasm between the haves and the have-nots.

And they are being buoyed by high-level support.

The lack of wage growth in a historically tight labor market is fueling the frustration people are feeling, said Heidi Shierholz, chief economist at the Department of Labor in the Obama administration and now at the left-leaning Economic Policy Institute.

And the decline of unions over the years plays into wage stagnation, she said, as does the Trump administration’s rollbacks of labor protections.

But the current booming labor market allows workers to take more risks, knowing there are so many job openings, Shierholz said. And the divisive rhetoric coming from the White House is similarly spurring people to band together.

The recent union successes have also built on a growing hunger for social justice fueled by movements such as Black Lives Matter and #MeToo, said Randy Albelda, an economist at the University of Massachusetts Boston. Women and people of color have figured prominently in the recent strikes, she said, and it’s helping people relate to union actions, which in the past have predominantly featured white men making good wages.

During the teachers’ strikes in parts of the nation, parents and students joined picket lines, knowing that teachers were trying to improve schools as well as their jobs; during the grocery store strikes, customers held signs alongside workers, realizing that “if the person who works at the deli can take care of her kid, that’s probably good for me,” Albelda said.

“It’s about the coming of age, in some ways, of the labor movement,” she noted.

The Marriott workers also experienced a “we’re in this together” mentality during their six-week strike last fall, said Carlos Aramayo, financial secretary treasurer of Unite Here Local 26, which represents hotel workers in Boston.

“I have never seen the level of public guy-on-the-street support that I saw during the Marriott campaign,” he said. “Just random people walking by and being like, ‘You guys are doing a good job, keep it up. You’re fighting for all of us.’ ”

The public’s heightened involvement in labor disputes is also being magnified by social media, including shoppers who posted photos of receipts from other grocery stores on Facebook to show they were boycotting Stop & Shop.

Still, unions must continue to evolve to embrace a changing workforce and the changing nature of work, labor analysts said. And unions need contracts that go beyond members’ jobs to improve education, health care, and other aspects of their communities.

“Democracy is about everyone getting a voice,” said Aramayo. “You need a voice at the ballot box, sure, but you need a voice at work.”

Katie Johnston can be reached at katie.johnston@globe.com. Follow her on Twitter @ktkjohnston.