William Jefferson Clinton was under the knife only seven weeks ago.
By Adam Brookes
BBC correspondent in Washington
Quadruple heart bypass surgery has left him thin and drawn. He looks all of his 58 years.
Democrats hope even a weakened Bill Clinton can aid John Kerry
But in Philadelphia on Monday, he took the stage next to the Democratic challenger for the presidency, Senator John Kerry.
As he looked out over an enraptured crowd, you could see a flicker of the old Clinton - one of America's most formidable and magnetic political campaigners.
"If this isn't good for my heart, I don't know what is," he said.
Mr Clinton's speech was brief. Apparently, he tires easily.
He rattled off a series of statistics that he said showed how the American economy had suffered during four years of a Bush administration - more Americans without jobs and health care, fewer educational and after-school programmes for disadvantaged children.
He ticked off some of the key policies of the Kerry campaign platform, and he dwelled on the Democrats' opposition to the Bush administration's programme of tax cuts.
Hoping for fizz
All this felt slightly cursory. But when he began talking about the big ideas, about America's role and posture in the world, his rhetorical gift returned for a moment.
A Kerry administration, he said, would bring back "a country and a world of shared responsibilities... where we co-operate when we can, and act alone only when we need to".
Democrats tend to sound wistful when they think of Mr Clinton's ability to transfix a crowd, to work a room, to speak in pithy, memorable sentences, and, above all, to look and sound warm.
Mr Clinton may be an ex-president who was disgraced and mocked for his dalliances while in office... but he remains an object of fascination in America and worldwide
Their candidate this year tends towards the lugubrious. Electricity is in short supply during a John Kerry speech.
The Democrats are hoping that bringing Mr Clinton out onto the campaign trail - even in his weakened state - will add some fizz to their campaign in these vital closing stages.
Mr Clinton may be an ex-president who was disgraced and mocked for his dalliances while in office.
He may have had a directionless four years since he relinquished the White House; he has achieved little in that time but pen a voluminous and not very highly regarded memoir.
But he remains an object of fascination in America and worldwide. He makes good television and he generates copy.
Boost in battlegrounds?
Beyond that, the Democrats have specific objectives in putting him on the stump.
First, they want to remind American voters of the roaring 90s - a decade of prosperity that saw home ownership rise, poverty decline, and a huge budget surplus accumulate.
It was on Bill Clinton's watch, say the Democrats, that America was last in the black. They hope voters will contrast their memories of the 90s with the much more patchy growth of the last four years.
Second, Bill Clinton has always enjoyed good poll numbers among African-American voters, and among women.
Given how close this election race is proving to be, these two constituencies, if they can be mobilised to vote, could help tip the vote in favour of Mr Kerry in the battleground states.
African-Americans are a particularly vital Democratic constituency.
The Joint Center for Political and Economic Studies, a Washington think-tank, points to significant African-American voting blocs in, among other states, Florida, Michigan, Ohio and Pennsylvania. All of these are battlegrounds.
"The size of the black turnout and the direction of black votes will be crucial in determining the next president," writes the centre.
Simpler times, despite scandals
The Republican Party - many of whose members retain a boundless loathing for Bill Clinton and his wife, Hillary - put out a well-crafted sneer as Mr Clinton took to the stump.
Party chairman Ed Gillespie told CBS television that Mr Kerry was hoping for "a little charisma transplant" from the former president.
On a more serious note, some Republicans argued that putting Mr Clinton out on the trail made the Democrats look like they were fighting the last election all over again.
It is true that Mr Clinton is no longer a day-to-day political player in America. But he reminds people of an earlier, simpler, pre-9/11 time - when the greatest source of political contention in America was the president's anatomy and the uses to which it was put.
Given the powerful, disorienting effect of terrorism and war on this country, the last Democratic administration does seem like a good time to many Americans. And Mr Kerry is now suggesting that he can bring it back.