Supporters of a proposed ballot measure to increase the District's minimum wage to $15 an hour rally at the John A. Wilson Building in Washington on April 15. (Aaron C. Davis/The Washington Post)

The D.C. Council unanimously agreed to boost the city’s hourly minimum wage to $15 on Tuesday, and Mayor Muriel E. Bowser pledged to sign the measure into law, lifting pay for low-income workers to among the highest in the country within four years.

The developments marked a victory for unions, which targeted the nation’s capital for a symbolic win in the “Fight for $15” campaign in a presidential election year.

Polls find strong support for a $15 wage floor as many Americans have become frustrated by the loss of well-paying manufacturing jobs and the growth of low-paying retail and service jobs.

Sen. Bernie Sanders of Vermont has tapped into that frustration in his presidential bid, calling for a federal $15 minimum, while Democratic front-runner Hillary Clinton has said she would support a $15 minimum wage if it is implemented gradually. Republican presidential candidate Donald Trump has alternatively said both that wages are too high and that “people have to get more.”

The District’s move is the latest in a series of unexpected and rapid-fire victories for the $15-minimum-wage movement. What began as an audacious push by fast-food workers just a few years ago is evolving into a new labor standard, with state lawmakers in California and New York agreeing to implement a $15 minimum wage by 2022 and legislatures in Connecticut, Massachusetts and New Jersey weighing similar measures.

The District has approved raising the minimum hourly wage to $15. Here's how the measure will be implemented. (Claritza Jimenez/The Washington Post)

Supporters, mostly Democrats, say a $15 floor is needed to help close the country’s growing income gap, especially in big cities. The District has the greatest income disparity between top earners and those on the lowest rungs compared with any of the 50 states, according to recent federal data analyzed by the city.

“When I see how much it costs to live in Washington, D.C. — and that cost is only going up — we know that it takes more money for every household to be able to afford to live,” Bowser (D) said on Tuesday, flanked by labor leaders and some business groups that begrudgingly said they would accept it. “There are families working day in and day out, sometimes two or three jobs but barely making ends meet.”

According to one estimate, the measure would mean a raise for 70,000 janitors, parking attendants, dishwashers and others, and it probably would put upward pressure on the wages of 44,000 more workers who are paid slightly above the new baseline.

But critics say a $15 minimum will be an order of magnitude larger than any previous wage floor and could prompt businesses to lay off workers, reduce their hours or increase automation such as self-serve kiosks.

James Sherk of the Heritage Foundation warned that the District might lose jobs in restaurants, hotels and other service industries. Walmart announced this year that it would not open two planned stores in the city in part because of high labor costs and other expenses.

“D.C. is only a few square miles. It will be relatively easy for businesses to relocate,” Sherk said. “People can stay in hotels in Arlington, they can go out to dinner in Alexandria. . . . There could be a migration of jobs from the District.”

Speaking in one of Washington’s poorest neighborhoods Tuesday, House Speaker Paul D. Ryan (R-Wis.) said the city’s decision would “actually do more harm than good in so many instances, because what it does is it prices entry-level jobs away from people.”

Economists across the political spectrum say there is little certainty about how a $15 minimum wage will affect low-income workers, and the businesses that rely on them, especially when the wage is introduced relatively quickly in the coming years.

“The bottom line is, we don’t know what impact this is going to have because we don’t have data on changes of this magnitude, this rapidly over time,” said Michael Lynn, a professor at Cornell University who has studied the effects of minimum-wage increases over the past 20 years. “The question is how tolerant and resilient businesses will be, and my fear is that this is dangerously fast.”

A $15 floor in the District would also set up one of the most stark geographical contrasts in the country. Across the Potomac River in Virginia, low-wage workers are paid the federal minimum of $7.25 an hour, which has been unchanged in seven years and is the standard in 20 other states.

A think tank backed by the hospitality industry released a survey last month warning that 1 in 5 businesses would consider leaving the District for Virginia.

Bowser said she was attuned to those concerns but said that as long as the city remained “hip” and “safe,” “people are going to want to live and work in the District of Columbia.”

She dismissed Ryan’s criticism that a high minimum wage would kill entry-level jobs. “We’ve heard that argument before,” she said.

The District’s hourly minimum, now $10.50, was already scheduled to rise to $11.50 in July. Under the measure passed Tuesday, it would continue to increase by about 70 cents a year until it reaches $15 in 2020. After that, annual increases would be automatic and tied to inflation, as unions have wanted.

The D.C. Council’s action, which still needs to be confirmed by a final vote, appeared to forestall a November ballot measure that would have raised the minimum wage to $15 for all workers, including waiters and others who receive tips.

To assuage the District’s powerful restaurant-industry lobby, D.C. lawmakers agreed to raise the base pay for tipped workers by a smaller amount, to $5 an hour from $2.77 an hour over the same period. After that, it would be tied to inflation. Employers in the District would remain responsible for paying employees the difference between their base pay and the minimum wage if tips do not make up the gap.

The discrepancy in pay for tipped workers drew criticism from a progressive bloc of the council and national advocacy groups for restaurant workers. But those lawmakers already are girding for the next labor fight: a proposal to add a 1 percent tax on employers to give workers in the District up to 16 weeks of paid family and medical leave.

Elisendo Morales, 30, an immigrant from Mexico who wore his janitorial union’s purple “Fight for $15” T-shirt, applauded the mayor during her announcement about the minimum wage. As he left the stage, he said he’d be back to lobby for paid leave.

“My mother is having surgery next week, and I need time off to take care of her,” he said.