Massachusetts is sticking to its guns on clean air, Gov. Mitt Romney (R) announced this morning. The state refused to extend a deadline for heavily polluting power plants to reduce their emissions, meaning they’ll have to clean up their acts by 2004. In 2001, then-acting Gov. Jane Swift (R) imposed the deadline on the state’s so-called Filthy Five power plants, ordering them to reduce emissions of nitrogen oxides and sulfur dioxide by 50 percent. Since then, the owner of one plant, Salem Harbor, has pressured the state to delay the requirement, and the state Department of Environmental Protection seemed ready to concede. But in a surprise development that is cheering environmental and health advocates, Romney stuck to the earlier deadline. He blamed Salem Harbor for 53 premature deaths, 570 emergency room visits, and 14,400 asthma attacks each year.