OLYMPIA, Washington (Reuters) - A voter initiative to enact a $15 minimum wage for thousands of workers in a Seattle suburb that houses the region's main international airport won a narrow victory on Tuesday that proponents hailed as a signal moment in the nationwide fight for livable wages.

The measure mandates that some 6,300 workers at Seattle-Tacoma International Airport and nearby hotels, car rental agencies and parking lots receive a minimum hourly wage more than double the federal minimum wage of $7.25.

Washington state's hourly minimum wage is already higher than any other U.S. state, and will rise by 13 cents to $9.32 an hour in January. The new wage in the city of SeaTac would be among the nation's highest, just below a $15.38 rate mandated for city workers and contractors in Sonoma, California.

Backers of the SeaTac wage ordinance see it as an opportunity to help local workers while encouraging other communities - particularly cities with progressive tendencies and smaller voting pools - to take similar action.

"It shows that people are tired of waiting for corporate CEOs or Congress to deal with income inequality and that they can use democracy to make a change," said Heather Weiner, spokeswoman for the union-backed Yes For SeaTac campaign.

The measure won by a margin of 77 votes with about 6,000 ballots cast, and King County election officials certified the outcome on Tuesday after weeks of uncertainty.

Opponents who fear the measure will slow the region's economy and drive businesses away said they plan to request a manual recount, for which they will have to foot the bill unless the result is reversed.

"This is a pretend solution to a really serious national economic problem," said Don Stark, spokesman for Common Sense SeaTac, a business-backed campaign opposing the initiative. "It is taking money from one pocket and putting it in another."

Foes of the measure, among them Alaska Airlines, have already sued to block it from taking effect in January, arguing in part that the city lacks the authority to impose a minimum wage on the airport, which is owned by the Port of Seattle.

A DRAWN OUT CONTEST

The election has been a drawn out affair. On election night three weeks ago, early returns showed the measure enjoyed an 8 percentage point lead, and supporters declared victory.

But Washington state requires voters to mail in their ballots or deposit them in drop boxes. As votes trickled in, the lead narrowed to as few as 19 votes over a week ago before expanding as problematic ballots initially set aside by election officials were tabulated.

The measure covers workers in the travel and hospitality industries, and provides sick leave in addition to a higher wage floor. It exempts small firms, airlines and unionized work forces.

While organized labor hopes SeaTac will act as a catalyst for similar efforts elsewhere - including in Seattle, which this month elected to the City Council Kshama Sawant, a socialist whose platform centered on a $15 minimum wage - the initiative is not without precedent.

Since 1994, when Baltimore instituted the country's first so-called living wage ordinance, more than 120 local governments have followed suit, according to the National Employment Law Project.

Four major California airports operate under ordinances similar to the SeaTac measure, including one guaranteeing workers at San Jose airport $13.82 an hour plus health insurance, and another mandating that Los Angeles airport workers earn $10.91 per hour plus health benefits.

(Editing by Cynthia Johnston and Jackie Frank)