Fast-food employees and other low-wage workers will take to the streets in Minneapolis and other major cities Wednesday to call for a $15 minimum wage, another step in a social movement that began three years ago to press for the biggest jump ever in the wage.

A group called Fight for $15, led by a national committee in New York and backed by some of the nation's biggest unions and progressive organizations, has drummed up support for higher wages and unionization through a combination of well-timed demonstrations, social media and on-the-ground recruiting.

Wednesday's demonstrations are billed as some of the biggest yet and they have spread to cities outside the United States in a manner similar to the Occupy movement in 2011. The wage-boost effort has lasted longer, though, in part because of its focus on a tangible outcome.

The federal minimum wage has been $7.25 an hour since July 2009, when it jumped up from $6.55 a year earlier. Since its inception in 1938, the longest the minimum wage held steady was nearly a decade from September 1997 to July 2007.

Minnesota last year became one of 29 states to set a higher minimum wage than the national standard. ­Minnesota lawmakers set the state's minimum wage to rise in three consecutive years, with the second jump taking place on Aug. 1 to $9 an hour from $8, and the third happening next year to $9.50.

McDonald's and Wal-Mart both announced wage increases this year. In February, Wal-Mart said it would increase pay to at least $9 per hour this month. McDonald's said it would set minimum pay at $1 over the local minimum wage for company-owned stores, beginning in July.

Activists say those changes show their campaign is working, though they're not yet ­satisfied.

"I think it's really showing that people are really unified around this issue, that they can achieve gains and achieve a living wage," said Lucas Franco, a Ph.D. candidate in political science at the University of Minnesota who has conducted research on minimum-wage campaigns in the U.S.

But market forces are also at work. Across the nation, unemployment levels have fallen and job vacancies have risen, putting pressure to end wage levels that have been stubbornly stagnant during the recovery from the 2008 recession. Since 2009, inflation-adjusted average pay in the United States has risen only slightly. Even the $9 an hour promised by McDonald's and Wal-Mart amounts to just $18,700 per year.

Activists in the Fight for $15 movement are seeking a more than doubling of the national wage, an unprecedented jump. The biggest change in the history of the minimum wage came in 1950, when it rose 88 percent to 75 cents an hour from 45 cents.

The pressure is occurring despite little change in the makeup of the low-wage workforce, which is 4 percent of the nation's hourly-paid workers. Half of the 3.3 million Americans who were paid at or below minimum wage in 2013 were under age 25, according to the Bureau of Labor Statistics. A generation ago, in 1988, 56 percent of the people working at or below minimum wage were under age 25.

Support for lifting the minimum wage ballooned in recent years, driven by instant Internet communication and intensive recruiting in cities.

Local activist groups like the Centro de Trabajadores Unidos en la Lucha send members from restaurant to restaurant to strike up conversations with employees about their working conditions.

Stephanie Gasca, a CTUL organizer, said she often asks workers for their names and wages before asking how they can survive on that little. Many of them, like Amanda Merritt, say they're struggling, she said.

"It takes a very emotional toll on me," said Merritt, a 20-year-old Burger King employee whom Gasca recruited to participate in Wednesday's strikes. Merritt earns $8 an hour, and management often changes her work schedule, she said. Her rent takes up most of her paycheck, and she wants to quit but can't find another job.

"I don't like feeling stuck," she said.

Besides face-to-face recruiting, the movement has relied on social media and the Internet to take what began as an isolated strike of fast-food workers in New York City and create a multinational push.

"This is a global movement of low-wage workers who are fighting to beat the fast-food industry … so that they can create a domino effect and raise wages for low-wage workers," said Kendall Fells, a New York-based national organizing director.

The group hopes to also lift wages for people like airport workers and adjunct professors. "Lecturers, like fast-food workers, are creating most of the value for the corporation or for the university," said Erin Trapp, a U lecturer, adding that current wages aren't enough to make ends meet.

Tyler Gieseke is a University of Minnesota student on assignment for the Star Tribune. Staff writers Adam Belz and Evan Ramstad contributed to this report.