Just days after businessman Tom Knox took himself out of Philadelphia's 2015 mayor's race, former Republican City Councilman Frank Rizzo Jr. has joined the Democratic Party and launched a trial balloon.

"It's obvious that there's a mayor's race and I'm considering entering that primary as a Democrat," Rizzo said at the Democratic City Committee headquarters, where he got party chairman U.S. Rep. Bob Brady to make his coming out a media event. "My experience as a 16-year veteran of city government, I think, prepared me well and there's a lot of things I can contribute to the city of Philadelphia."

We've been here before. Rizzo has talked about running for mayor several times before, and never followed through. Back then, he would have had to resign his City Council seat to run. Now that he's out of office, he doesn't have to take that risk.

His father's son

This balloon may not float for long, but Rizzo is an interesting figure for a couple of reasons.

His political career is derived from his famous name. If you're young, you don't remember his father, but he was a force of nature. Big, powerful, charismatic, and controversial, he was a tough-as-nails police commissioner in the 1960s and a two-term mayor in the '70s. He kept running to regain the mayor's office until his death in 1991.

He always had a political base, mostly in white rowhouse Philadelphia, and he was one of the most racially polarizing figures of of modern urban politics. But his edges softened as he got older and times changed, and, when he died, his followers believed he had significant black support in the city.

But Frank Jr., "Franny" as he was known for years, cut an interesting political course. He constantly invoked his father, whom he refers to as "my dad," but he had a different profile in the city's racial politics. He was a Republican, but when Mary Mason, for decades Philadelphia's leading black radio talk show host, was on vacation, it was Franny who took her chair and spent the morning with black Philadelphia.

He served four terms in City Council, and I give him credit for taking the job seriously. He got into a number of policy issues in depth, and was known for delivering constituent service (and being tough on his staff). He was also very responsive to media queries and would speak his mind and take a position, something many in Council are terrified of doing.

Rizzo lost his seat in 2011 because he strayed too often from Republican orthodoxy and lost the party's backing -- and because he cashed in on the widely detested DROP city pension program.

A man whose time is past?

If Rizzo really decides to have a go at the mayor's office, it will be a steep climb.

There was always a loyal Rizzo following in this town, but I have to wonder how many of those folks are still around. A good poll will offer some insight into that. By the time Rizzo runs in 2015, it will have been 24 years since his father ran for office, and 40 since he won an election (but only eight since Rizzo Jr. won a Council seat).

And Rizzo himself will be 72 when the Democratic primary is held. He's a young-looking 72. In fact, when I was clean-shaven, I would get mistaken for him on the street -- happened several times. He says he's in great health, but his age and his association with an earlier generation of politics could be a drag on the effort.

But  Rizzo does have one thing going for him. There are at least a half-dozen candidates in the mayoral  mix at this point, and anything can happen in a multi-candidate primary. Few political pros gave Michael Nutter much chance a year and a half before the 2007 mayoral primary, but he proved them wrong.