Bill Clinton has appeared at a rally alongside John Kerry in a bid to boost the Democratic candidate's chances in next week's US presidential election.
Clinton appeared to relish being back on the stump again
The ex-president, who has spent two months recovering from heart surgery, told Pennsylvania voters that Mr Kerry had a "better plan" for America.
Republicans have said the decision to "roll [Mr Clinton] off the surgery table" smacks of desperation.
Neither Mr Kerry nor George W Bush have a clear lead in the opinion polls.
"If this isn't good for my heart, I don't know what is," Mr Clinton joked as he took the microphone in the city of Philadelphia.
He depicted an ailing economy in Pennsylvania and the wider US, compared with the situation when he was in office.
On foreign policy, he said Democrats wanted a world of "shared responsibilities and shared opportunities".
Speaking about Mr Kerry, Mr Clinton said he was the kind of man who wanted people to "think and hope".
Taking the stage after Mr Clinton, Mr Kerry berated the Bush administration for allowing nearly 350 tons of explosives to go missing in Iraq.
Earlier on Monday, in New Hampshire, he had accused the Bush administration of "unbelievable blindness, stubbornness and arrogance".
In Pennsylvania, he stressed again his support for stem cell research, promising that cures for illnesses such as diabetes would be pursued.
The state is one of a handful which could be won by either candidate and may decide the outcome of the election.
Influential American weekly The New Yorker has come out in favour of John Kerry in its editorial, praising his "fairness of mind" and "independence of spirit".
"Kerry," it says, "offers a clear, corrective alternative to Bush's curious blend of smugness, radicalism, and demagoguery."
Mr Clinton's appearance in Philadelphia will appeal to some undecided voters and African-Americans, says the BBC's Justin Webb.
Mr Clinton was controversially excluded from the 2000 Democratic presidential campaign after his last years in office were marred by a sex scandal.
However, he remains the Democrats' greatest star, our correspondent says.
Terminator for Ohio
Mr Kerry's team has been paying tribute to Mr Clinton's abilities as a veteran campaigner who galvanises crowds with his impassioned manner.
However, advisers to President Bush seized on his appearance as a mark of weakness.
Bush had former New York Mayor Rudy Giuliani at his rally
"The fact that John Kerry's going to have to roll him off the surgery table and onto the campaign trail demonstrates... that he's underperforming in key parts of his own constituency," White House communications director Dan Bartlett told the Fox News network.
The sitting president launched a fierce attack on Mr Kerry on Monday, accusing him of "a strategy of pessimism and retreat" on Iraq.
"My opponent has the wrong strategy, for the wrong country and
the wrong time," he told a rally in Greeley, Colorado.
In a dig apparently also aimed at Mr Clinton - who bombed militant bases in Afghanistan with missiles in 1998 - he suggested the Democratic candidate would try to fight terrorism "with subpoenas and a few cruise missiles" .
The Republican campaign will seek to win over the vote in the swing state of Ohio this weekend when California governor and Hollywood action hero Arnold Schwarzenegger will speak up for President Bush.