The summer before a contested primary traditionally has been the time when a field of candidates starts to coalesce.
The list of names weighing the contest to succeed Mayor Nutter is long, and the number of eventual entrants could top the five who ran in 2007.
But most insiders say this year's slate remains far more fluid than in 2006, failing to generate the same buzz of that year, when virtually all of the city's political titans were poised to run.
Unlike 2006, when Nutter resigned his City Council seat in July, jump-starting the mayoral campaign, no serious contender has taken the plunge this summer.
That may be thanks partly to Clarke, the Council president, who has been circumspect about his plans, said veteran Democratic consultant Larry Ceisler.
"He's sort of the logjam," Ceisler said. "I think a lot of that depends on what Darrell Clarke does."
Clarke, a frequent Nutter foil, would be a formidable candidate and fund-raiser - he's a favorite of John J. Dougherty, who holds the levers of Local 98, the electricians' union, and a political juggernaut.
But Clarke is private by nature, and some observers wonder about his desire to face the scrutiny of a mayor's race. Clarke, unsurprisingly, declined to describe his plans last month.
Instead, he issued a statement that said in part: "The reality is that right now, my focus is not on talking about what my future may hold but on doing the best job I can as the Council President of Philadelphia."
If he doesn't run, his supporters and donors are up for grabs.
Clarke, like Butkovitz and Councilman James F. Kenney, is a city elected official, required by the City Charter to resign before running for another office.
Given the amount of fund-raising and organization needed, conventional wisdom says the sooner a candidate declares, the better.
Kenney said in July that he was "engaged in a thoughtful process" about the mayor's race - but added that he had the "experience, compassion, and the passion" to be mayor.
The son of a Philadelphia fire chief, Kenney speaks to rowhouse Philadelphians while also being known as a champion of progressive economic causes and immigrant and LGBT issues. Recently, he successfully sponsored a bill to end arrests for marijuana possession, and he's been prodding the mayor to sign the legislation.
Kenney, however, has said he's concerned about giving up his Council seat - and his income - to run.
"It's a natural thing to think about," he said in an interview. "It wouldn't be the deciding factor."
In any case, Butkovitz, the city controller, said the race won't truly heat up until after the November gubernatorial election.
Butkovitz said he had been "talking to people all over the city" about the mayor's race.
"Although I can't commit at this stage, it's becoming increasingly likely that I'll get in," he said, adding that he probably would defer if Clarke ran.
As controller, Butkovitz is known to many voters, even if his government watchdog role isn't well understood by the public.
At his inauguration to a third term this year, Butkovitz signaled a possible campaign theme, speaking about the city's growing income inequality - a strategy that worked for New York Mayor Bill de Blasio.
Amid all the speculation about who is running here next year, the name of State Sen. Anthony H. Williams has been the most consistently uttered.
Williams flirted with running against Nutter in 2011, but ultimately backed the incumbent. Williams also ran for governor in 2010, finishing third in a four-way Democratic primary.
His father, the late State Sen. Hardy Williams, ran for mayor in 1971 against Frank L. Rizzo and was among the first generation of independent black leaders.
Through a spokesman, Williams declined to discuss whether he would follow his father's footsteps and be a candidate next year.
One potential wild card in the race - and a formidable one, at that - is former District Attorney Lynne Abraham, who told The Inquirer last week that she's "certainly in the mix."
Abraham was elected the city's top prosecutor five times, and she's remembered for her tough-on-crime reputation.
"I love this city and loved serving it. I think we're poised for greatness," she said. "There's no reason, given the issues of the city and how well I know those issues, that I shouldn't consider it."
Abraham said she had a time frame to make a decision, but she wouldn't elaborate. Rivals could point to her age - 73 - but she said: "It's not a matter of chronological age. It's where your heart and your head is."
Former Gov. Ed Rendell, too, has people urging him to run again, 15 years after he left the mayor's office. As unlikely as that seems, Rendell - a mere lad of 70 - was coy in a July interview, saying he was focused on the fall gubernatorial race.
People "stop me in the supermarket, on the street, and say they wished I would run for mayor," he said. "It's all very flattering, but I'm not going to think about it until after November."
Gillen and Trujillo, both respected political insiders little known to the public at large, seem poised to enter the race. Neither has declared, but both are setting up their campaign apparatuses. And either, if they won, would claim a first: Gillen would be the city's first female mayor; Trujillo, the first Latino to lead the city.
They could be hoping to win with a Nutter-esque strategy of appealing across a wide spectrum of voters, rather than relying on powerful constituencies, such as organized labor.
Trujillo, a lawyer who has been the city solicitor, a federal prosecutor, and a businessman (he owns a Spanish-language radio station and website), declined to comment.
Gillen, who served in Rendell's and Nutter's administrations, said she's taking a lesson from Tom Wolf, the virtual unknown who captured the Democratic gubernatorial nomination in May.
"Voters are inclined to support people who are not career politicians," she said.
Then there's Frank Rizzo Jr., the former Republican councilman and son of the legendary mayor, and T. Milton Street, the former state senator, former federal prison inmate (for failing to file his tax returns), and older brother to former Mayor John F. Street.
Neither would seem to have a chance to win, but they could peel votes from those who do. Kenney even accused Rizzo of being "a shill" for Williams - Rizzo then took and passed a lie-detector test as part of a Philadelphia Magazine profile to prove that any mayoral run he makes is on his own initiative.
"Like my father used to say, I'm waiting in the high grass, watching what everyone is doing," Rizzo said last month. "If I think the field is favorable for me, I'm in. But I'm not going to do this just to wear out my clothes."
Milton Street, the only Democrat to challenge Nutter's 2011 reelection, drew 24 percent of the primary vote by promising to represent society's "don't-counts." He thinks he can mobilize that vote again.
"Am I serious about running? Is fat greasy?" he said. "I'm not thinking about whether I can win. . . . I want the candidates to have to address the issues of second-class Philadelphians."
Oh, and let's not forget Sam Katz, the three-time GOP mayoral candidate who nearly beat John Street in 1999 and who is keeping his options open. This time, he says, if he runs, it won't be as a Republican.
"I've been thinking about running for mayor since I was about 7," he said in July. "My thinking about running is not news."
With such a diffuse field, what's going to separate one candidate from another? Ceisler said the last three mayors - Rendell, Street, and Nutter - had one thing in common.
"They all really wanted it," the political consultant said. "They really had that burning desire in the gut."
Who has that drive this time remains to be seen.