The question of how long and how low CNN ratings can slip before a change at the top was answered late last week as the network’s worldwide prexy Jim Walton announced plans to step down.

Walton will leave at year’s end and it’s likely a shakeup is on the way. TBS chief executive Phil Kent will lead the search for his successor at the news net, which has struggled to find a primetime hit and whose centrist politics have been steadily eclipsed by Fox on the right and MSNBC on the left. May was CNN’s worst month for total viewers in primetime in 20 years with ratings down more than 50% year on year.

Time Warner chairman Jeff Bewkes has defended CNN Worldwide publicly as an overall profitable venture and one that tends to dominate big breaking news stories. But at some point, lagging ratings threaten to overshadow the core brand and Bewkes is said to have become increasingly concerned in recent months. Both he and Walton acknowledged the need for change.

“I respect him personally and support the decision that he and Phil Kent have made,” Bewkes said in a statement Friday.

Walton, a 30-year CNN vet who has headed the news operation for the past 10 years, said in a memo to staffers that the cabler ”needs new thinking” and ”a new leader who brings a different perspective, different experiences and a new plan.”

Kent in a statement praised Walton for his stewardship since 2003. He credited the exec with having the vision that ”modernized and globalized our legacy news brand, enhanced CNN’s journalistic standing (and) positioned it at the forefront of multiplatform branded news content.”

CNN has made some moves of late, hiring popular Travel Channel host Anthony Bourdain to host a “No Reservations”-like show on the weekends and appear on air and online. It continues to struggle with primetime as well as shows leading into the evening hours. A 6 pm program with political reporter Jon King was canceled last month.