Does he really mean it this time?

Millionaire Tom Knox, who finished second to Michael Nutter in the 2007 mayor's race, has announced through spokesman Frank Keel that he plans to run for mayor in 2015 and has formed a political committee for the campaign.

Knox's declarations of candidacy are oddly at once credible, and not credible.

They're credible because if he really gets in and funds his own campaign, he's a top-tier candidate. When I spoke to him Friday morning, I asked what he'd spend to run for mayor.

"The simple answer is, whatever it takes," he said, reminding me that he came up with serious cash for his 2007 run. "I put over $10 million of my own money into the race, and if I need to put that much in again, I'll do it."

But Knox also has a credibility problem, stemming from the false starts in his political career since 2007.

He said he was running for governor in 2010, then backed out. He took a poll and considered running for mayor in 2011, but didn't. And, just a few months ago, he assured reporters he was running for governor next year. I still have the "Knox for Governor" business card he gave me.

Now he says it's the mayor's race. So, as Regis used to say, "Final answer?"

Absolutely, Knox said.

"I've got my wife on board. I've got my kids on board," he said. "They all say I'd be better off not running. But this is something I have to do."

Knox talked about the need to fix the schools, and he's convinced he can find ways to save big in the school district budget. He thinks he can put 50 mini-public health centers around the city at no additional cost to taxpayers.

But one question arises. Knox is 72 now. If he wins, he'd be 75 a month into his first mayoral term, and nearly 80 when he finishes it. Is he up to this?

Again, abosolutely.

"I'm probably in better health than most of the guys that you'll see getting into this race," he said. "I go to the gym five or six days a week. I could probably sit down and do more pushups than anybody you know."

(A pushup contest would make a fun addendum to the first debate. Tony Williams, start training).

Knox's age suggests that if he's going to win elected office, he can't put it off much longer. So maybe he means it this time. He said he'd have a formal announcement soon, and would add a professional campaign consultant I'll be impressed with.

All of that is reversible, of course. In the 2010 governor's contest, he said he was running and had credible campaign help who were quickly dumped when he changed his mind.

Knox was a deputy mayor under Ed Rendell in the early '90s.

One more note: Under a clause of city campaign rules, if Knox puts $250,000 of his own money into the race, the city contribution limits for all candidates will double. Under this so-called millionaire's rule, businesses and political committees could give up to $23,000 to a candidate.