Harris Gruman, a top official with the Service Employees International Union in Massachusetts, was riding an old Red Line car somewhere beneath the city of Boston when he got the call.

It was a warm day in the spring of 2014, and the union was two years into an intriguing bid to rethink organized labor. Standard unionization remained a priority, of course. But the SEIU was also reaching beyond its membership, organizing thousands of non-union workers and folding them into a new coalition with left-leaning faith and community groups called Raise Up Massachusetts.

In a matter of months, the coalition had collected a thick stack of signatures for a ballot measure designed to establish the highest minimum wage of any state in the country. On the phone that afternoon was a key lawmaker looking to cut a deal: What kind of wage hike would the Legislature have to approve, he asked, to make the referendum go away? It was then that Gruman realized how quickly this one new group had knitted itself into the fabric of power on Beacon Hill.

It’s an article of faith within the labor movement, and across much of the political left, that forming unions is the best way for workers to secure a better living. But for reasons ranging from deindustrialization to stiff employer resistance, traditional organizing has long been a struggle; today, just 11 percent of American workers are unionized.

The emerging face of worker advocacy is “alt-labor,’’ a broad suite of creative workarounds that includes Raise Up-style coalitions and nonprofit “worker centers’’ pressing for legislation, filing lawsuits, and launching media campaigns on behalf of the hard-to-organize. The National Domestic Workers Alliance has helped push a “domestic bill of rights’’ through a series of state legislatures, earning director Ai-jen Poo a MacArthur “genius’’ award a couple of years ago.

And the Fight for $15, which began as a far-fetched bid to raise the minimum wage for fast-food workers in New York City, has become a national phenomenon — catching fire with graduate assistants and airport workers, and winning tangible victories in Seattle; Washington, D.C.; and California.

Now, with an incoming Republican president widely expected to make traditional unionization even more difficult, alt-labor faces its biggest test: Can the fledgling movement provide a new template for pro-worker activism and, more broadly, for a beleaguered American liberalism in the age of Donald Trump?

Big Labor, once suspicious of the movement, is betting on it. The AFL-CIO’s alt-labor wing, Working America, now claims 3 million members. And the SEIU is backing the Fight for $15 push and experimenting with state-level coalitions like Raise Up Massachusetts.

Unions, of course, have long partnered with like-minded advocacy groups to fight budget cuts or rally around a particular issue. But Raise Up is designed to stick around, a permanent collaboration that takes seriously the organizing chops of the unions’ religious and community partners. Not coincidentally, it’s emerged as perhaps the most important new political force in Massachusetts, taking a lead role in addressing the state’s startlingly sharp divide between rich and poor.

After that spring 2014 call to Gruman, the state Legislature passed a gradual increase in the minimum wage, topping out at $11 per hour next year. And Raise Up won voter approval of a paid sick leave measure that same year.

As alt-labor activists see it, these efforts amount to a re-imagination of the traditional union contract. The goal is to push the negotiation out of the shrinking realm of unionized labor and into the political sphere, where advocates can win gains for hundreds of thousands of workers who are not formally organized.

“Raise Up is about being, essentially, a union for the million lowest-income workers in Massachusetts,’’ Gruman said. And after the gains of a couple of years ago, “it’s time to negotiate a new contract — that’s what a union does.’’

Now, the coalition is leading the next big fight in Massachusetts politics: a so-called millionaires’ tax, slated for the 2018 ballot and soaring in public opinion polls, that would raise billions of dollars for education and transportation. Gruman says paid family and medical leave and another minimum wage hike are also on the agenda.

The effort has alarmed business interests and antitax groups that see Raise Up as a mere front group for unions like the SEIU and the Massachusetts Teachers Association and warn that its agenda will ultimately slow economic growth.

That kind of concern is particularly potent in red states, posing a dilemma for an alt-labor movement with national ambitions. “I think it’s a challenge,’’ said Karen Nussbaum, executive director of the AFL-CIO’s Working America. Yes, many pocketbook issues have an appeal that crosses party lines: Voters in conservative Arizona just approved paid sick leave. And last year, Working America helped win a minimum wage hike for city workers in Greensboro, N.C.

Still, alt-labor has a lot to figure out. Much of the movement is focused on the black, Latino, and immigrant communities at the heart of the Obama coalition. Some of its leaders remain convinced that a continued focus on this emerging electorate makes sense even now.

“We think our strategy and our theory is right,’’ said Roxey Nelson, an SEIU political director involved in a Raise Up-style coalition in Florida that sought to drive up minority turnout in the presidential election. “And because of who we represent, we are in the right communities for our membership and our base.’’

Nelson says alt-labor needs to find ways to talk to Trump’s white working-class supporters, too. But it’s not clear that a movement steeped in the language of multiculturalism can speak to these voters in the way that unions once did. “This election shows, in part, what happens when there’s a decline of institutions that organize white working-class people and give them a feeling of voice and power,’’ said Janice Fine, a Rutgers labor professor.

Even if alt-labor makes a conscious effort to fill that void, its impact may be curtailed by its size and strength. Fine’s research shows that “worker centers,’’ one major slice of the alt-labor sector, have expanded from five in 1992 to about 240 today. That’s real growth, for sure, but the movement remains relatively small.

Many of these centers are shoestring operations at the mercy of foundation grantmakers. And even those with backing from Big Labor can’t feel terribly secure, given the parlous state of union finances nationwide. A pressure campaign called Our Walmart, which has advocated for wage hikes and more full-time work at the huge retailer, sustained a heavy blow when the United Food and Commercial Workers cut support for the effort last year.

Nelson Lichtenstein, director of the Center for the Study of Work, Labor, and Democracy at the University of California Santa Barbara, adds that the gains made through boycotts and legislation can fade without a formal contract and a union to enforce it. “Yes, you can win benefits — and you do it by law, that’s a good thing,’’ he said. “But without an actual union, those things become fragile. You may be defeated in the next election.’’

Without a union, he adds, there is a decline in the sort of labor consciousness that can drive change in the workplace and in politics; in Michigan, a Rust Belt state where Trump made enormous inroads, Hillary Clinton still won union households by 16 percentage points.

Nevertheless, worker advocates probably shouldn’t count on a resurgence of traditional unionism at this point. And alt-labor, whatever its vulnerabilities, seems a movement well-suited to the post-industrial age: stocked with service workers and driven by spectacle.

On Tuesday, the Fight for $15 movement plans a wave of demonstrations in 340 American cities, sure to be filled with brass bands, giant puppets, “McPoverty’’ signs, and other social media-friendly totems of 21st-century dissent.

In Massachusetts, organizers say, there will be a fast-food protest in Cambridge, a rally at Logan Airport, and a “major legislative announcement at the State House.’’

Gruman of the SEIU says advocates will file legislation for a $15 minimum wage. And they’ve got a plan if lawmakers are unwilling to move. The Raise Up coalition has collected hundreds of thousands of signatures for ballot measures in the last few years, and it’s prepared to wield the clipboard again.

David Scharfenberg can be reached at david.scharfenberg@globe.com. Follow him on Twitter @dscharfGlobe.