Instant polls conducted by US television networks gave a clear edge to the Massachusetts senator after a showdown on Thursday with Bush on foreign policy and national security issues that was dominated by Iraq.
CBS showed Kerry the winner by 44%-26% and ABC by 45%-36%. A CNN/Gallup poll put the margin at 53%-37% and said 46% reported feeling better about the Democrat after the debate, compared to 21% for Bush.
Analysts agreed that neither Kerry nor Bush landed any heavy blows during the 90-minute encounter at the University of Miami, the first of three presidential debates before the 2 November election.
"This was the closest thing to a tie", said Larry Sabato, director of the University of Virginia's Center for Politics. "I really was impressed. Kerry was articulate and crisp for once and Bush, I've never seen him so articulate."
But Kerry managed to deliver his most cogent attack yet on the Iraq war and convey a sense of solidity and coherence in front of the man who has spent months ridiculing him as a weak-kneed waffler.
"I think he was very effective at downplaying the argument that he had flipped and flopped on different issues," said David Corbin, a poltical science professor at the University of New Hampshire.
It was not clear whether Kerry's showing would translate directly into the new support he badly needs to overcome Bush's lead in the polls, generally estimated at five to eight points.
Observers say Kerry may have
given a boost to his supporters
But several analysts said he now had a chance of recharging the somewhat dispirited Democratic ranks and winning back anti-Bush voters who had deserted him amid doubts about his leadership.
The debate was make-or-break for Kerry, whose campaign had been reeling from relentless Republican assaults on his credibility, toughness and vaunted combat record in the Vietnam war.
Iraq key issue
Both sides predictably claimed victory.
The Republicans said Bush effectively highlighted the mixed signals the Democrat was sending on Iraq. They said the president made a compelling case for more time to finish the "war on terror" he started after the 11 September 2001 attacks.
Democrats hailed their candidate for hammering away at the war in Iraq as a diversion from the real battle against al-Qaida and building his credentials as a smarter, more effective commander.
Kerry held Bush responsible for
ordering a muddled invasion
Kerry also tried to cover confusion over his various votes in the Senate on Iraq by acknowledging that he might have flubbed some of his rhetoric, but Bush's muddled invasion was much worse.
According to some analysts, while Bush started strong in holding himself out as the best hope for keeping the United States safe, he appeared to lose steam and showed some irritation towards the middle of the debate before closing with a flourish.
Charles Jones, a senior fellow at the Brookings Institution, called it virtual draw that could work in Kerry's favour. "It seems to me has has demonstrated in this area that he is capable and that may stop the haemorrhaging."
Stephen Hess, a former Republican speechwriter now with the Brookings Institution, thought the debate would help firm up some of Kerry's support that had become wobbly.
"There may be some gain on the part of people who were not as familiar or aware particularly of Kerry who now get a sense of his seriousness and articulateness," Hess said.
Jonathan Siegel, a law professor at George Washington University who has done volunteer work with the Kerry campaign, thought that Kerry had given a boost to the Democratic rank and file.
President Bush has previously
described Kerry as a waffler
"I think a lot of people who have been a little exasperated with his campaign will have some renewed vigour and say, 'Listen he's finally up there fighting.' During August you sort of got the sense he was sitting there and taking it."
Siegel had no illusions that the debate itself would erase Bush's lead and propel Kerry to the front. But he added, "I think this would prevent people from closing the deal with Bush.
"This leaves the matter open. It shows we are still in a presidential race and there is a lot of time before election day," he said.