WASHINGTON — Senate Republicans have asked environmental regulators to use their power to halt the country's plans to expand ethanol production amid rising food prices. Twenty-four Republican senators, including Senator McCain of Arizona, sent a letter Friday to the Environmental Protection Agency suggesting it waive, or restructure, rules that require a fivefold increase in ethanol production over the next 15 years. Congress passed a law last year mandating a ramp-up to 15 billion gallons of corn ethanol by 2015 and 36 billion by 2022.