In what’s being billed as one of the biggest enforcement actions in U.S. history, the federal government yesterday sued seven big electric companies in the Midwest and the South, accusing them of violating clean air rules. The companies own 32 coal-fired power plants in 10 states, and the feds say they have been upgrading the plants without adding the modern pollution controls required by the Clean Air Act. The government says that if the plants cut their pollution as required, it would have the same effect as taking 26 million cars off the road. Enviros welcomed news of the lawsuits, as did officials in Northeastern states and New York, which suffer from air problems exacerbated by pollution that drifts in from the Midwest and the South. The utilities denied making major upgrades to the plants in question, saying the changes were only routine maintenance that didn’t require the addition of pollution controls.