Knox, a millionaire who lost to Nutter in a five-way Democratic mayoral primary in 2007, made the announcement at City Hall, with Nutter at his side. He repeated his earlier statement that he had switched his affiliation in April only on the outside chance that Nutter lost last month's Democratic primary to long-shot candidate T. Milton Street. Nutter won easily.
Knox said he was announcing his switch back to Democrat in an effort to end speculation that he might challenge Nutter.
"I'm not running for mayor at all," he said. Instead, Knox said he intends to run for governor in a few years. Republican Gov. Tom Corbett was inaugurated in January and potentially would face re-election in 2014.
Former Mayor John F. Street, Nutter's predecessor and longtime political foe, also switched his party affiliation from Democrat to independent but has not said whether he would run for mayor.
At this point, Nutter faces only Republican candidate Karen Brown in his bid for a second term. Brown, a former schoolteacher, is the clear underdog in a city where Democrats outnumber Republicans more than 6-to-1.