The e-mail arrived at 11:38 yesterday morning - about 12 hours after Democratic voters chose Michael Nutter as their nominee for mayor.

"Not that this will come as a surprise to anyone," wrote Sam Katz to a reporter, "but I will not be a candidate for mayor in the general election."

Katz, a three-time candidate for mayor, raised that prospect late last month when he quietly changed his registration from Republican to "non-affiliated," a move that set him up to run as an independent on the November ballot.

But after Nutter's win, Katz says he's out.

"I just think the guy did a virtually unbelievable job on every front," said Katz, who stopped by Nutter's victory party Tuesday night at the Radisson Plaza - Warwick Hotel. His son, a Nutter campaign volunteer, had invited him.

"It was the first time I'd been in a room with a lot of Democrats in a long time," quipped Katz, a Democrat until he became a Republican in 1990.

Saying he and Nutter had too much in common as far as issues they push for improving Philadelphia, Katz said he was never going to consider a race against Nutter.

Rather, while not directly saying so, Katz made it clear that his party registration switch was motivated by the possible election of businessman Tom Knox.

"Look, the truth is," Katz said, "it was an insurance policy against the possibility that Philadelphia could have wound up with something that was not a positive force." He declined to go into details.

Katz said he didn't know whether he would support Nutter in the general election, or Republican nominee Al Taubenberger.

Pointing to Nutter's across-the-board success - Nutter captured more votes than candidates Chaka Fattah and Bob Brady in key parts of their own backyards - Katz said: "It was impressive and gives himself a real mandate to do whatever it is he wants to do."

Contact staff writer Marcia Gelbart at 215-854-2338 or mgelbart@phillynews.com.