Stepping into Cohen's City Hall office, if not quite his shoes, will be Rost, the low-profile deputy mayor for policy and planning.
As Rendell's most trusted adviser, Cohen has been described on occasion as the ``co-mayor'' in tribute to his stamina, intelligence and effectiveness.
Yet his departure has prompted no fear within the administration that the sky is falling. Rost, 39, may be unknown to the public, but his colleagues say he has what it takes to handle the job second only to the mayor's in the day-to-day running of America's fifth-largest city.
``You need somebody who is smart, talented and has good judgment,'' says Michael Masch, former city budget director. ``And there's nobody else who has those attributes and who has also worked at the highest policy levels, who is a good administrator and knows such a broad range of issues.''
``He is probably,'' says Dean Kaplan, current budget director, ``the quickest study among us.''
A strapping 6-foot-3-inch former Ph.D. candidate in government and administration, Rost has worked for Rendell since signing on as a policy adviser in the 1991 mayor's race. As nice a guy as ever crushed your hand in saying hello, he is talked of in terms normally reserved for Sara Lee.
``I don't know anybody in the government who doesn't like Greg Rost,'' says Kevin Feeley, the mayor's spokesman.
That affability should prove an asset in a role that frequently requires saying no to department heads, Council members and citizens who might approach with quite reasonable, but overly expensive, requests.
``David was able to do that in a way that didn't engender personal animosity,'' says Masch, now budget director at the University of Pennsylvania. ``Absent Cohen, nobody is better than Rost in that role because of his temperament.''
For several years, Rost has been the anonymous chief author of some of the administration's most important documents: the 50-page introduction to the annual Five-Year Financial Plans and the mayor's annual budget address. They have been cogent, fact-filled reports on the city's current situation and the administration's goals.
The deadlines hit back-to-back in early January. ``My holidays are always ruined,'' Rost says.
Not that he's complaining.
``I enjoy seeing government work,'' Rost says with a glint in his eye. ``And I enjoy city government most. It's where the rubber hits the road. More than in Washington or in Harrisburg, you can really see the impact of what you do.''
Much of the stylistic and rhetorical tone of the administration, therefore, has come from Rost. He's been a key figure on school district issues and in state and federal relations - nailing the details, for instance, in the city's gaining hundreds of Crime Bill cops.
Rost was interested in government even while growing up outside Baltimore. His father, a corporate executive, subscribed to a slew of newspapers. Rost, a voracious reader, loved politics and the sports section. His career choice was sealed, he said, when ``I found I couldn't run the 100 in 9 [seconds] flat and couldn't dunk a basketball.''
After graduating from the University of Maryland, Baltimore, and working temporarily as a carpenter, he dived into a doctorate program at UM, College Park. On a whim, he transferred to an MBA program for policy types at the Fels Center of Government at the University of Pennsylvania - a fateful move, as it turned out.
At Fels, he met a classmate who would become his wife (Marcy Feldman of Cherry Hill), became deputy director of the program and met the people who introduced him to Ed Rendell.
Swept up in Rendell's fervor to revive the distressed city, he joined the issues staff for the l991 campaign. When Rendell won, the issues shop moved into the government as a kind of in-house think tank, providing the intellectual backing for an array of issues that changed the way the city governed itself: labor negotiations, insurance, fleet management, energy costs.
Like many of the top managers, Rost worked long hours. But he never felt so rewarded. ``Public service is a very high value with Greg,'' says his wife, Marcy Feldman Rost, a planner at the Delaware River Port Authority.
``We worked our brains out,'' recalls Masch, ``And from the very beginning, the tone from the top was, `We're here for public service.' ''
``I have this awful, gnawing feeling,'' Greg Rost confides, ``that nothing I ever do in life will be as exciting as those early years of the Rendell administration.''
That sentiment may change after a couple of years in the demanding chief-of-staff job. Cohen recently made up a list of his duties. It came to 70 to 80 items - two pages worth, single-spaced.
Rost won't do all that Cohen did. ``I can't out-David David,'' he said when his appointment was announced in February. Nor will he have to. Many of the chores Cohen took on will be splintered off to others.
Cohen, for example, did much of the talking for the administration. Now Feeley and Rendell will answer reporters' queries more often. Cohen's old role in economic development will be taken by Rost and Finance Director Ben Hayllar. Budget Director Kaplan will take even greater responsibility for the budget.
Rost will chair what Cohen calls ``really the steering committee for the government'' - the Initiative Compliance Committee. The group oversees productivity efforts and polices spending, seeing to it that departments spend less than budgeted. This practice, called ``targeted budgeting,'' has been a key to the administration's four straight years of budget surpluses.
And Cohen will still be a phone call away, a temptation that's mocked in a sign in Rost's Municipal Services Building office: ``In Case Of Emergency, Call 1-800-DLC.''
Cohen won't be figuring out who gets the tickets to the mayor's box at the Vet, ``but on any mega-issue, David will still have input,'' according to Rendell. After meetings of the Initiative Compliance Committee, for example, ``four or five of the most difficult issues will be brought to his attention for his input,'' the mayor said.
If there are any questions about Rost, it is whether he will be able to play the disciplinarian. ``People are always asking me, `Will you have any problem saying no?' '' Rost says.
``I tell them, `No,' '' he said, to prove the point.
If Rost needs the backup, Rendell has made it clear he'll crack the whip with any recalcitrant department head.
``I won't hesitate to pick up the phone and say, `Get your ass in here, I want to talk to you,' '' Rendell said in an interview earlier this year.
`` `Look, you know what targeted budgeting entails. You did it for five years. You do it a sixth year, or it will be early retirement or back to civil service for you.' ''
With Cohen so close by - Ballard Spahr Andrews & Ingersoll is only two blocks west of City Hall - and so often consulted, won't Rost feel overshadowed? Second-guessed?
Will he feel like Chief of Staff Lite, the real Chief of Staff being just offstage?
Rost says not.
``I'm a team player,'' he says. ``Whatever we can do to help Ed Rendell run Philadelphia is fine in my book.''