Edens adds that he is "confused by" Southern's decision to hire union workers, explaining that, as he sees it, "any project done by union labor always comes in behind schedule and over budget."

Mississippi Power—like SCS, a Southern Co. subsidiary—continues to believe the May 2014 on-line date "remains achievable" despite challenges, says Mississippi Power spokesman Jeff Shepard. "We anticipate the continued utilization of union labor force throughout the completion of the project."

Louie Miller, director of the Mississippi chapter of the Sierra Club and a leading critic of the Kemper project, says Mississippi Power has repeatedly understated the magnitude of management, staffing and other problems at the site.

"This is Southern's Waterloo," Miller says, noting the project is now more than $1 billion over budget. Additional cost overruns are likely, he adds.

Kemper has certainly grabbed the attention of Southern executives. On May 20, Mississippi Power President and CEO Ed Day abruptly retired and was replaced by Ed Holland, Southern's general counsel; Tom Anderson, Mississippi Power's vice president for generation, also suddenly left the utility.

In April, Southern took a $333-million write-off to reflect Kemper cost overruns it cannot pass on to ratepayers.