Massachusetts Acting Gov. Jane Swift (R) unveiled regulations this week that will make the state the first to limit carbon-dioxide and mercury emissions from power plants. The rules, which will go into effect in June and apply to the six dirtiest plants in the state, will also require big cuts in nitrogen oxide and sulfur dioxide emissions. The rules are also the first in the country to prohibit plant owners from averaging emissions reductions at more than one plant to satisfy requirements. Swift noted that she had chosen a different path from President Bush, who last month broke a campaign promise to regulate CO2 emissions from power plants: “He and I, in this case, came to a different conclusion.”