With the federal minimum wage stuck at $7.25, the battle for better wages has moved to the state and local levels. Workers on the front lines celebrated one of their biggest victories yet this week when the Los Angeles City Council approved what may be the highest minimum wage in the country for hotel workers. But passage of the $15.37 hourly rate also highlights the challenges unions face in preserving jobs and growing wages as they target specific areas and industries.

The rate will likely apply to only 39 of the city's 350 hotels because of exemptions. It could also wind up backfiring in the short term by triggering layoffs at affected hotels and creating an "ill-fated competitive difference" with nearby exempt businesses, said Bob Amano, the executive director of the Hotel Association of Los Angeles.