Clinton's influence on Kerry began before he joined him on the campaign trail this week.
After critics said he was too slow to respond to Swift Boat ads that attacked his war record, Kerry hired several former Clinton aides and advisers -- including Democratic strategist and commentator James Carville and former White House press secretaries Joe Lockhart and Mike McCurry -- to reinvigorate his campaign. During the presidential debates, Kerry avoided the long-winded, perhaps overly intellectual answers that had become his trademark (and a punch line for comedians) and relied on shorter, more direct replies that arguably spoke more to his audiences' emotions or issues they could relate to.
In addition, Kerry has attacked Bush more aggressively. From American casualties in Iraq to record high oil prices to the flu vaccine shortage and this week's reports of hundreds of tons of explosives gone missing in Iraq, Kerry has reacted almost instantly to headlines and news flashes that show potential weaknesses in the president's administration. Bush, in turn, accused Kerry of making "wild charges" without knowing all the facts.
"I think one of the things that [Kerry campaign advisers] Mary Cahill and Bob Shrum were slow to realize was the people tend to vote on their gut feeling about a candidate, not necessarily their public policies," Westen said. "The Abu Ghraib prison scandal, the unrest in Iraq [in July and August] came and went with Kerry hardly saying anything. Now, Kerry's had a complete transformation, attacking Bush on the weapons depot and using words like 'incompetent.'"
Clinton's campaign efforts, especially in the wake of his recent heart surgery, may also help Kerry by giving him a courage infusion.
Opponents have labeled Kerry a "flip-flopper" because of his vacillating -- or often complex -- stances on issues such as the Iraq war and gay marriage. The Swift Boat ads challenged Kerry's honesty, courage and convictions and his relatively slow response to the attacks didn't help him.
"Clinton's campaigning for Kerry so soon after heart surgery could be seen as courageous, and any association he [Kerry] has courage can only help him," said Westen.
However, despite being a decorated war hero, Kerry still faces challenges to his leadership skills, bravery and beliefs. Clinton's support helps, political observers say, but Kerry himself must convince undecided voters that he can lead the country in a time of terrorism fears and economic uncertainty.
"In the end, John Kerry's going to have to win this election by himself," ABC News Political Director Mark Halperin told ABC News Radio. "It's up to John Kerry to do his own heavy lifting. And if Bill Clinton helps focus the spotlight on him, that's what the Kerry campaign's looking for."
If Kerry's bid for the White House fails, critics say it will not be because of Clinton. The former president will remain an asset to his party and his mystique will still be intact.
"If Kerry loses, it will be despite Clinton, not because of Clinton," Taylor said.