After weeks of blogging about the Philadelphia mayor's race, Sam Katz has taken a step toward jumping into it.

The three-time mayoral candidate has left the door open for a fourth run, quietly switching his party registration at the eleventh hour last week from Republican to independent.

In an interview yesterday, Katz repeatedly declined to say if he was considering a run. But he did not rule it out, saying only: "It is what it is. I wouldn't read too much into it."

Katz's move leaves him eligible to run as an independent in the fall general election if he can get enough signatures on nominating petitions. Under city rules, any independent candidate would need 1,967 signatures to get on the Nov. 6 ballot.

His switch is also the first tangible sign that the race for mayor may not end with the May 15 Democratic primary.

Date-stamped records filed with the county board of elections show Katz's paperwork was dropped off on the last day possible - April 16 - to meet the pre-primary voter-registration deadline.

"Son of a gun," said Bob Lee, who oversees voter registration for Philadelphia, as he pulled up Katz's new party registration on a city computer.

While no independent has ever been elected Philadelphia mayor, the closest to do so was Charles Bowser. In 1975, Bowser, a lawyer, came in second to Frank L. Rizzo but ahead of the Republican candidate that year, Tom Foglietta - who later went to Congress as a Democrat.

"Are you serious? Oh, my goodness," said lawyer Carl Singley, Katz's 2003 campaign co-chair, when told last night of Katz's switch.

Singley sized it up this way: "It probably represents the notion that if Tom [Knox] is the [Democratic] nominee, that Sam is in position to tap into the disenchantment of the voters of the other four candidates."

Knox, a millionaire businessman, has forged ahead of rivals Bob Brady, Dwight Evans, Chaka Fattah and Michael Nutter in the five-way Democratic primary race. But Singley, who supports Evans, said neither his candidate nor the other non-millionaires should be counted out. "This thing is still pretty fluid," he said.

Democratic consultant Larry Ceisler said Katz may be banking on an independent candidacy's having broader appeal than his last race - in 2003, when he ran as a Republican against Mayor Street and lost.

"Maybe his calculation, if he wants to run, is that he'll get his Republicans, he'll get the Democrats he had before, and now he'll get the Democrats who wouldn't vote for him because he was in George Bush's Republican Party," Ceisler said last night.

But Ceisler predicted the Democratic candidates this year would each have wide appeal to Philadelphia voters in November. "I just don't see where the votes come from. He's not running against John Street anymore."

Katz has called the sole, party-backed Republican mayoral candidate, Al Taubenberger, "a fine guy." But he has also expressed doubt that a Republican can win with the 2008 presidential election around the corner and Democrats' desire to keep Pennsylvania a blue state.

Though Katz showed himself to be a prolific fund-raiser in his earlier elections, he would be restrained by the same Philadelphia campaign-finance caps that have limited the amount of fund-raising for this year's primary - aside from the millions Knox has put into on his own campaign.

Katz was a mayoral candidate in 1991 and 1999 as well as 2003. He came within a whisker of defeating Street in 1999. Now a managing director for an investment company, Katz is writing about this year's campaign as an online blogger for Philadelphia Magazine.

"I've never been good on the sidelines or on the bench, and it has been painful at times lately," he wrote in a posting dated April 2. "Tigre Hill's movie, The Shame of a City, is omnipresent - I get to watch or hear constantly about my getting ripped off in 2003 and then merely observe a race in which I would have hoped to have been standing for re-election. Honestly, I can't say that I'm loving it."

Katz has hosted screenings of the documentary film, which follows him through his failed 2003 campaign, to help repay some of the million-dollar debt he still has from that race. Almost half of that is money he lent himself that he said he won't collect.

On a blog posting just last Friday, Katz called this year's race "as listless a campaign as any I can remember."

Michael P. Meehan, chief of the city's Republican Party, called Katz's party switch "an interesting development."

Meehan added: "I thought from everything up to this point that he was done with electoral politics. But maybe he's not. Very interesting."