Proving the adage that there are no permanent enemies in politics, former Mayor John F. Street has been encouraging his two-time Republican rival, Sam Katz, to run for mayor next year in the Democratic primary, according to sources close to the men.

Katz has told a number of political, labor, and community leaders that he is considering a challenge to Mayor Nutter, and the flirtation has become a hot topic of conversation in political circles.

But the apparent alliance with Street seems shocking considering the bitter campaigns they fought against each other. Street beat Katz narrowly in 1999 and then by a larger margin in 2003, a race that centered on charges that Street was corrupt amid a federal investigation of some members of his administration.

"They respect each other intellectually, get together for lunch about once a month, and talk fairly often," one source said of Street and Katz. Street wants Nutter to lose, knows a Republican can't beat him, and sees Katz as a viable Democratic challenger, the source said.

"The issues facing Philadelphia are significant," and Street and others in the former mayor's orbit believe "there should be significant debate," the source said.

Street and Nutter have had a sometimes-rocky relationship, and the incumbent has been sharply critical of Street's role as chairman of the Philadelphia Housing Authority board. Federal authorities are investigating secret settlements of sexual-harassment suits against PHA Executive Director Carl H. Greene, as well as allegations that agency managers and vendors were asked to solicit money to honor Greene.

Neither Street nor Katz responded to requests for comment Friday. Nutter's spokesman, Doug Oliver, declined to comment.

Several sources who have spoken with Katz recently said that he had not made a decision. He has been producing a documentary on the history of Philadelphia.

U.S. Rep. Bob Brady, also chairman of the city Democratic Party, said he met with Katz about six weeks ago. Brady described it as a courtesy call requested by Katz, who told him he was considering the race.

"I told him I'm supporting the mayor," Brady said. "In fact, I gave Michael Nutter a fund-raiser Tuesday night."

Katz is responding to a "certain dissatisfaction" with Nutter among some business leaders who think the mayor has not been bold enough in trying to change city government, said a source who has spoken with Katz.

A new poll sponsored by the website, out Friday, found that Nutter would have a narrow lead of 38 percent to 34 percent over Katz in a hypothetical matchup, with 28 percent undecided.

Nutter's prospects are better in a multicandidate field, with up to 32 percent of the vote. The survey, conducted by the firm Municipoll, tested several well-known political figures: Brady; U.S. Rep. Chaka Fattah; State Rep. Dwight Evans; businessman Tom Knox, who ran in 2007; City Councilman Bill Green; and City Controller Alan Butkovitz.

The automated telephone poll is based on responses from 810 registered Democrats who said they were likely to vote in a 2011 primary. Results are subject to a margin of error of 3.4 points.

Katz changed his registration from Republican to Democrat on March 31, 2008, returning to his political roots. He joined the GOP in 1990 because the Democratic Party would not give him a chance to run for office.

He lost in the 1991 mayoral primary, and also unsuccessfully sought the GOP nomination for governor in 1994 before going on to carry the party standard in the two mayoral races.