A few months back, you'd hear political gossip that, deep down, Mayor Rendell really wanted Sam Katz to succeed him in office. Rendell would go through the motions of supporting City Council President John Street, the buzz went, but he secretly welcomed a Katz victory.

Nobody says that anymore.

As national chairman of the Democratic National Committee, Rendell does not plan to see his city fall into Republican hands. He's thrown himself into the Street campaign with money, advice, TV commercials, direct mail, a presidential visit and soon, some personal appearances.

Rendell has had to overcome his publicly declared distaste for negative campaigning, which the Street campaign has embraced with gusto for weeks.

The mayor says he thinks the negative ads have been issue-oriented, rather than personal attacks "so I'm less uncomfortable with it. But I don't care if he does it - I won't do it."

Although Rendell isn't doing attack ads, it's clear his competitive juices are flowing. Speaking at a leadership dinner Wednesday night, he said Katz was "pandering" with his pledge to replace School Superintendent David Hornbeck.

It was clear Rendell had a passion for this race in September, the day after Democrat John F. White Jr. endorsed Republican Katz. Rendell didn't wait for reporters to ask about Street's charge that White had made a deal with Katz out of self-interest.

"You know, there's no question there was a deal," Rendell told reporters. "I would bet my right arm that if Sam Katz becomes mayor, you're going to see John White in some high place."

When Katz finally revealed his wage-tax plan, David L. Cohen, Rendell's friend and former chief of staff, led the attack, excoriating Katz's plan with data supplied by Rendell administration officials.

Rendell has continued to host Sunday-night strategy sessions at his East Falls home.

He used his national connections to secure an upcoming visit from President Clinton, and the Street campaign plans to call voters after that with a recorded message from the president.

Polls show Rendell is still popular and suggest that voters care what he thinks. But how much difference will it make?

"It's a positive," said Gerry Kosinski, Democratic leader of the 45th Ward in the lower Northeast. "Voters in the end always make up their own minds, but it doesn't hurt.

"And I think the Street campaign will really play the Ed card in the last week," Kosinski said. "They're building toward a crescendo, like a fireworks display with a big finish."

Indeed, Rendell has already done taping for new TV spots. He'll mail a letter to voters with a personal message. He promises to do some public appearances and step up the heavy fund-raising he's already done for the campaign.

"I'm the clean-up guy," Rendell said Wednesday. "I'm the guy who calls and says, 'Hey, Dave, you're invested in the campaign for $15,000. We need five more down the stretch.' "

Rendell has made relatively few personal appearances with Street, and some Democrats grumbled that he and Street should have spent the summer doing small groups in liberal and middle-class neighborhoods, expected to be swing votes Nov. 2.

Rendell did a lot of that in Street's 1995 Council race against challenger Julie Welker, attending coffee klatches in the Center City portion of his district.

Rendell said the Philadelphia mayor's race could have national implications if Pennsylvania becomes a key state in a close presidential election.

Asked if it would be an embarrassment to lose this race to a Republican now that he's become a national Democratic figure, Rendell shrugged.

"Sure. It's not great," he said. "But there's never been a race for mayor between a black Democrat and a white Republican that hasn't been close."