The bill signed into law by Seattle Mayor Ed Murray on Tuesday to raise the city’s minimum wage to $15 an hour by 2021 is historic. It will more than double the current federal minimum wage of $7.25 — something no local government has ever done.

An estimated 46 percent of Seattle’s 100,000 workers who make under $15 an hour will earn that by 2018. The measure could pump $3 billion into the local economy over the next decade, cutting poverty by 30 percent. It is already resonating, as $15-an-hour campaigns are underway in Chicago, Los Angeles, Portland, and San Francisco. New York Gov. Andrew Cuomo just endorsed a plan that could raise the minimum wage in New York City to $13 an hour.

Seattle’s new law has downsides, however, including a seven-year timeline, a lower training wage aimed at teenagers, weak enforcement provisions that encourage wage theft, and allowances for businesses with fewer than 500 employees to count tips and healthcare benefits reflected on pay stubs as wages up to $3 an hour.

But workers elsewhere who flip burgers, fold sweaters, and pull shots of espresso for a living wonder if the victory in Seattle can be replicated. That depends on understanding how it was won. There was no guarantee a year ago that the “$15 an hour” slogan would become law.

The driving force was Kshama Sawant, an avowed socialist who voters elected to the City Council last November on a platform to raise the minimum wage and taxes on the wealthy. She and her political party, Socialist Alternative, mobilized pressure for a wage bill. They fought opponents for months, parrying attempts to block the increase.

Given the obstacles they faced, a bill might have never materialized. Seattle is home to corporate titans like Starbucks and Amazon that have built their profit model on poverty-wage jobs, and during Sawant’s campaign the Seattle Times dismissed her as “too hard-left for Seattle.”

She and her partners made the most of a limited hand nevertheless. Sawant pursued a broad platform, but Socialist Alternative soon realized that $15 an hour should be the banner issue because of the amount of support it generated. The momentum became unstoppable after local newsweekly The Stranger gave Sawant a full-throated endorsement in September. Murray, who was running for mayor at the time, quickly endorsed the measure as well.

After taking office, Murray appointed a business-heavy Income Inequality Advisory Committee to devise a proposal. Sawant and Socialist Alternative countered by establishing 15 Now, a group that organized public marches and rallies, set up chapters in 11 neighborhoods, and canvassed widely.

Meanwhile, some Seattle restaurant owners pushed their staffs to oppose the $15-an-hour increase. Jess Spear, 15 Now’s organizing director, told VICE News that two prominent restaurateurs told their staff that prices would increase and that they would lose tips and even their jobs.

A group composed of servers and bartenders called Tips ARE Wages threatened to turn a key group of workers against the proposal. But Spear said that 15 Now met with the group and won them to their side after leaked documents revealed that restaurant owners were orchestrating the anti-$15 campaign.

But rallies and meetings are little match for billion-dollar corporations, which put small-business owners out front, moaning that jobs and business would be lost if $15 an hour became law. In March, Sawant and 15 Now made a strategic retreat by conceding that small businesses should have a longer phase-in period for the increase. At the same time, they deftly argued that businesses like McDonald’s, which amassed $5.5 billion in profit last year, could afford a much quicker timetable.

Meanwhile, 15 Now launched a ballot initiative to remind councilmembers that if they failed to pass a strong bill, voters could decide the issue for themselves. On April 26 it held a national conference in Seattle where attendees approved a plan that established a $15 an hour increase by 2017, with no tip credit or training wage. The conference was timed to anticipate the unveiling of Murray’s proposal on May 1. The Stranger noted that Murray’s plan was “so complicated reporters can’t understand it,” but it did take the wind out of 15 Now’s sails because it gave the impression that the battle had been won.

Labor leaders quickly closed ranks behind Murray.

“It’s a very delicately constructed deal and my advice to council would be to change nothing,” David Rolf, the president of the 40,000-member Service Employees International Union 775, told me in early May. He said that a ballot initiative would result if the measure were watered down — but that’s exactly what happened. The bill introduced on May 15 was weakened with a training wage, the tip/healthcare allowance, a three-month delay, and slap-on-the-wrist enforcement measures. Rolf still reversed course. “We fully support the ordinance,” he later said.

Socialist Alternative’s leaders privately conceded that they couldn’t win a ballot initiative without full support from organized labor, but they pushed one forward anyway to keep up the pressure on the council as it considered the ordinance. The City Council approved the bill on Monday afternoon, sending it to Murray’s desk for his signature.

Though the push for a $15 minimum wage is expected to go national, it’s unclear whether 15 Now will find similar success in other big cities. Sawant was aided by unusual factors like a non-partisan election and The Stranger’s backing. Even so, the proposal’s success in Seattle demonstrates a new model of grassroots resourcefulness that has the potential to widely advance a pivotal social agenda.

Follow Arun Gupta on Twitter: @arunindy