A massive legion of Democratic faithful packed several square blocks downtown Monday as former president Bill Clinton made a late and long-awaited return to the campaign trail with a rousing endorsement of Democratic nominee John F. Kerry.

Clinton's physical frame was notably thinner, and his voice, too, seemed to be showing the signs of a slow recovery from heart surgery last month. The raspy exhortations were familiar but weaker, as the 42nd president compared his record on jobs with that of the 43rd president, and pledged that Kerry would revive Democratic policy successes of the 1990s.

Four years after being effectively banished from campaigning by the 2000 Democratic nominee, Al Gore, Clinton beamed with obvious delight at being put to work this time. He and Kerry stood side by side amid roars from partisans.

"If this isn't good for my heart, I don't know what is," Clinton said. "From time to time, I have been called the Comeback Kid. In eight days, John Kerry's going to make America the Comeback Country."

Kerry's campaign regards Clinton as a potent tool for sparking enthusiasm and encouraging voter turnout among Democratic base voters. In recent days, Kerry strategists have been trying to balance multiple requests from battleground state operatives for his presence, against the limitation that Clinton's still-frail physical condition cannot accommodate a hectic schedule with several stops in a day.

Clinton flew late Monday afternoon to Florida -- where Gore was also campaigning, and urging Democrats not to forget the 2000 election controversy there. Clinton will be making appearances in the coming days in New Mexico and Nevada, Kerry operatives said.

His most dramatic appearance, however, will come on Sunday when he returns to his native Arkansas, concluding with a rally in Little Rock or Pine Bluff that evening. Arkansas went decisively for President Bush in 2000, but recent polls have shown the race tightening there.

Clinton's health scare -- an emergency quadruple bypass followed a diagnosis of dangerously blocked arteries -- has added an element of drama to these appearances. A Philadelphia fire department spokesman estimated the crowd at around 80,000, although some news organizations placed the number lower. In either event, it would be among the largest crowds Clinton has ever addressed in the United States, although he appeared before larger assemblages overseas.

The willingness to use Clinton illustrates how, at least among target blocs of voters, the personal scandals of his term -- which made his own vice president conclude that Clinton was a political burden -- have receded since 2000, while memories of the robust 1990s economy still echo.

"We have just lived through four years of the first job losses in 70 years, record bankruptcies, middle-class incomes declining and poverty going up," Clinton said. "In Pennsylvania alone, you've lost 70,000 jobs, as compared with the 219,000 you gained by this time when that last fellow was president -- me."

People began arriving in the early morning at the rally site, along Benjamin Franklin Parkway, in the shadow of such Philadelphia landmarks as the statue of William Penn atop City Hall and the LOVE sculpture. By noon, it felt like a festival with bright-colored signs, parents pushing toddlers in strollers and high school students dancing to music blared from the stage. The national anthem was performed by Philadelphia native Patti LaBelle.

Ricardo Franco, 20, a junior at the University of Pennsylvania who plans to cast his first presidential vote next week for Kerry, said Clinton's personal failings, which led to his impeachment in the Monica S. Lewinsky controversy, mean little to him.

"Bush's lies affected the entire world and are responsible for the deaths of many American soldiers, and even with those deaths, victory has not been assured," he said.

Lorraine Ziegler, who lives in Old Bridge, N.J., brought her two daughters, including 8-year-old Monique. "She missed school today for a more important lesson in life," Ziegler said. She said Clinton's coming out to campaign for Kerry is a "very powerful" statement.

In an interview with ABC's "Good Morning America," Clinton said his chest remains "a little tender in the morning" until he starts moving about, and friends have said he tires easily.

In Philadelphia, Clinton said Bush is relying on a fear campaign to save a troubled candidacy. "You know, I've been home watching it, all this stuff," he said. "The other side, they're trying to scare the undecided voters about Senator Kerry. And they're trying to scare the decided voters away from the polls."

To laughter from the crowd, he added, "It worked so well in Florida, they seem to be trying it elsewhere."

Gore also hit this theme in his Florida visit, at a rally with several hundred people gathered under the palm trees outside a Broward Community College library. Those angry about the 2000 race -- in which just over 500 votes separated Gore and Bush in Florida, in a race that ultimately turned on a Supreme Court decision -- should "turn those feelings into energy, determination for change," the former vice president said.

Florida began early voting -- for the first time in a presidential election -- starting on Oct. 18, and Gore urged people to go before Election Day: "That will give them plenty of time to count your vote. This time they are going to count every vote. We don't want the Supreme Court to pick the next president, and we don't want this president to pick the next Supreme Court."

Harris reported from Washington. Staff writer Manuel Roig-Franzia, traveling with Gore in Florida, contributed to this report.

Sen. John F. Kerry (D-Mass.) and former president Bill Clinton greet tens of thousands at a downtown Philadelphia rally.