Kerry lost his voice at one point in the hours before the Iowa caucuses as he strove to sway undecided Democrats.
But when the results came in, the Massachusetts senator scored a stunning victory with 38% of the vote, against 32% for fellow Senator John Edwards and just 18% for the favourite, former Vermont governor Howard Dean.
"We came from behind and we came for the fight," Kerry told cheering supporters at his state campaign headquarters in Des Moines as he took aim at drug firms and the oil industry.
"And now I have a special message for the special interests that have a home in the Bush White House: 'we're coming, you're going, and don't let the door hit you on the way out'."
Jubilant supporters toasted John Kerry's surprise victory late on Monday at a rowdy party at his hotel headquarters in Des Moines, last-minute converts mingled with dedicated grass roots supporters.
"He had the best organisation, it was just the least visible," said an elated and relieved Kirk Tofte, who has been a volunteer with Kerry's Iowa campaign since October.
The 60-year-old Vietnam war hero had trailed Dean for many weeks going into the caucuses, which launched the Democratic Party's nomination process.
Kerry hailed his supporters who engineered the turnaround.
The former Vietnam veteran
hailed his supporters
"Thank you, Iowa, for making me the comeback Kerry," he said, highlighting how "not so long ago, this campaign was written off.
"But in your homes in Iowa, in community centres, in Veterans of Foreign Wars posts, in restaurants where you never let me stop and eat, in homes, living rooms and barns where we gathered across this great state you listened, you stood the ground, and on caucus night you stood with me so that together we can take on George Bush and the special interests."
The victory will give new momentum to Kerry's campaign, for which only last month he had to mortgage his half of the family home in Boston to help raise more than six million dollars in new funds.
'Drug lobby or oil industry'
"In the months and years ahead, I pledge that I will be fighting for you and for all Americans across this country whose voices are being stolen by powerful interests.
"We need to join together all across this nation to restore fairness to our country," he declared.
"I am running to free our government from the grip of those powerful interests, whether it's a drug lobby or whether it's an oil industry," he said.
"I'm running so that you will have a president who is on your side, not on the side of those powerful interests who fuel campaigns in America. I'm running to restore a concept called fundamental fairness"
"I'm running so that you will have a president who is on your side, not on the side of those powerful interests who fuel campaigns in America. I'm running to restore a concept called fundamental fairness."
Kerry has become a vocal critic of the Iraq war even though he voted for a congressional resolution in November 2002 to give the US president permission to launch the invasion.
He has accused the Bush administration of running "the most reckless, arrogant, inept, and ideological foreign policy in modern history," and vowed to increase international involvement in securing and rebuilding Iraq.
Meanwhile, voters who celebrated with Kerry offered a glimpse of political pressures at play during Monday's voting.
New Hampshire is the next stop
for presidential candidates
Michael O'Keefe went to his caucus with the intention of backing Missouri representative Dick Gephardt - more out of sympathy than conviction.
"It was a toss-up between Gephardt and Kerry," said the 58-year-old. "I went back and forth. Kerry is the better candidate in my view, but I wanted Gephardt to get a decent showing in Iowa."
But his plans were thwarted by a rule that disqualifies any candidate that does not get 15% of voters in the room, and he wound up giving his support to his second choice.
"I bolted for Kerry immediately," he said, resisting the blandishments of activists working on behalf of Howard Dean.