The nation's largest public utility voted Thursday to close six coal-powered units in Alabama and replace two more in Kentucky with a new natural gas plant.

At the board meeting in Oxford, Miss., Tennessee Valley Authority CEO Bill Johnson said increasingly stringent environmental regulations and flat power demand have made it necessary to rethink how the utility generates electricity.

"This is a personal nightmare for me," said Peter Mahurin, a board member from Bowling Green, Ky., said of the decision. "But I must support what I believe to be in the best interest of TVA's customers."

In fiscal year 2013, coal accounted for 38 percent of TVA's portfolio while natural gas made up 8 percent. Johnson said he would like to see those numbers closer to 20 percent each over the next decade.

Senate Republican Leader Mitch McConnell met with Johnson last month to seek continued operation of all three coal-burning units at Paradise Fossil Plant in Drakesboro, Ky. The board had previously approved upgrading the two oldest units with environmental controls. But on Thursday, Chief Operating Officer Chip Pardee recommended building a gas plant there instead.

He said the third unit at Paradise is newer and has sufficient environmental controls to continue operating on coal.

The board also voted to close all five units at the Colbert plant in northwest Alabama and one of two remaining units that had not been marked for closure at the Widow's Creek plant in northeast Alabama.

Board member Joe Ritch, of Huntsville, Ala., echoed Mahurin's comments on the closures, saying, "As painful as it is, it's the right thing to do."

He said that saving a few jobs now would reduce TVA's competitiveness for years to come.