The Seattle city council voted to push up the city’s minimum wage to $15 an hour. If the wage hike is fully implemented, it will guarantee Seattle’s workers the nation’s highest minimum wage. The increase in the minimum wage will be phased in over a number of years. Big employers that do not provide their employees a health plan are the first that will face the $15 per hour minimum, a requirement that will be fully phased in around 2017. Large employers who offer health benefits will have to pay the $15 minimum starting in 2018. Small businesses with employees who receive tip income will have to pay the $15 per hour minimum a couple of years later, but the countable wage will include employees’ tips. By 2021 all employers in the city must offer a minimum wage of $15 an hour, regardless of the employer’s size.

The federal minimum wage is currently just $7.25 an hour, unchanged since 2009. If Congress does not raise the national minimum wage, Seattle’s minimum will be more than twice the federal minimum wage. Many states currently have a higher minimum wage than the federal one. As it happens, Washington has the nation’s highest state-level minimum wage, $9.32 per hour. Unlike the federal minimum wage law, Washington’s state law increases the state minimum wage every year in line with changes in the consumer price index. By the time Seattle’s $15 per hour minimum becomes effective for large employers in 2017, the Washington state minimum wage will be about $10 per hour, assuming consumer prices continue to rise 2% a year. Thus, large employers in Seattle will have to pay their minimum-wage employees 50% more than minimum-wage employees receive outside the Seattle city limits.

I strongly favor the Administration’s proposal to boost the U.S. minimum wage to $10.10 an hour. It will boost the spendable incomes of millions of poorly paid workers and their families, and I expect it will have only a small adverse effect, if any, on low-wage workers’ job opportunities and work hours. However, I am more cautious about the wisdom of raising a single city’s minimum wage to $15 an hour when nearby jurisdictions leave their minimum wages unchanged.

One reason for my caution is that a big minimum-wage hike can place Seattle’s low-wage employers at a competitive disadvantage compared with employers engaged in the same line of business but located in a nearby suburb. If compensation costs for low-wage workers represent a big percentage of a Seattle employer’s costs, and if the employer faces competition from businesses on the other side of the city limits, companies located in Seattle can lose customers to competitors outside the city.

Consider a business that mainly sells low-cost, fast-food meals. If it must pay $15 an hour to its low-wage employees, while its competitors less than a mile away are only required to pay $10 an hour, the companies outside Seattle can charge lower prices to their customers for shakes, burgers, and fries, and yet still make a profit. The lower cost establishments can capture a larger percentage of the local fast-food trade, reducing fast-food sales inside Seattle’s city limits. The same is true of the goods and services sold by laundry and dry cleaning establishments, inexpensive motels, and other businesses that depend on low-wage workers to stay competitive. The labor cost disadvantage caused by a higher minimum wage can hurt low-wage employment in Seattle and possibly reduce the value of some of the city’s commercial real estate.

To the extent that consumers have the option of buying goods or services from companies that are not required to pay a higher minimum wage, some of the hoped-for gains from a higher minimum wage will be lost. When customers can conveniently buy products or services from firms that face lower labor costs, the new businesses that they patronize will grow and the old, high-cost businesses they abandon will shrink. Low-wage workers may earn higher wages inside the Seattle city limits, but their employment opportunities in Seattle may shrink.

Seattle is a prosperous city, and its mayor was elected in part because of a promise to boost the pay of its most poorly paid residents. If a $15 an hour minimum wage has a chance of working and enjoying broad political support, Seattle is a good place to test the idea. I will be interested to see whether low-wage Seattle businesses continue to prosper even after they are required to pay a minimum wage that is 50% higher than the one faced by competitors in nearby suburbs. The risk of a big minimum-wage hike at the city level is that the city’s low-wage employers will be harmed in their competition with out-of-town businesses that sell the same products or services. The risk of this kind of harm is vastly smaller when the minimum wage is increased at the state or national level. If the Administration can persuade Congress to boost the national minimum wage, all employers—inside and outside a city’s limits—will be required to raise the pay they offer to their most poorly paid workers.

Note: An edited version of this post appears on Fortune.com