The party's national convention gave Kerry the nod late on Wednesday after hearing a vibrant tribute from running-mate John Edwards, who hailed him as a battle-tested warrior bent on wiping out years of Bush's "hateful, negative politics".
The nearly unanimous vote by more than 4300 delegates set the stage for Kerry's acceptance speech on Thursday, which could be a defining moment for his White House ambitions.
With polls showing voters still unclear about the character and policies of the four-term Massachusetts lawmaker, Kerry, 60, must make the speech of his life to counter Republican charges he is weak and waffling, analysts said.
The Democrats rolled out some heavy military brass to tout Kerry's leadership and decisiveness, two areas where surveys show Bush with a clear edge.
A dozen retired military officers, including a couple who called themselves Republican defectors, saluted Kerry's vision in a statement of endorsement, a video and speeches to the floor.
"I believe that no one will be more resolute in defending America nor in pursuing terrorists than John Kerry," said General John Shalikashvili, a former chairman of the joint chiefs of staff and newly converted Democrat.
John Edwards trumpeted his new
boss' history in Vietnam
Foreign-policy aides to Kerry also roasted the Bush administration for failures in Iraq and elsewhere. They pledged to reverse the president's go-it-alone approach and win back allies for Washington's so-called war on terrorism.
Susan Rice, a new senior adviser to the Democratic candidate, had particularly strong words on the war against Iraq.
"Iraq remains unstable and increasingly a magnet for terrorists from all over the world and perhaps the next new proving ground for these terrorists," she told reporters outside the convention here.
Kerry wrapped up a week-long, cross-country campaign push and flew into town on Wednesday, before crossing Boston's historic harbour by boat in the company of veterans he served with in Vietnam.
Running mate's words
Edwards, a smooth-talking populist senator from North Carolina, did his best on Wednesday in Boston to build up his new boss, mixing high praise for Kerry with biting broadsides against their opponents in the 2 November election.
Edwards, 51, also capped a concerted, daylong Democratic effort in Kerry's hometown to boost the decorated Vietnam war hero's credentials as a determined and tough leader.
Drawing the contrast between Kerry and Bush, who never saw combat, Edwards said, "When a man volunteers to serve his country, and puts his life on the line for others - that's a man who represents real American values.
He said Kerry's navy comrades "saw up close what he's made of. They saw him reach down and pull one of his men from the river and save his life.
"And in the heat of battle, they saw him decide in an instant to turn his boat around, drive it straight through an enemy position, and chase down the enemy to save his crew," said Edwards, who was to be formally anointed to the number two slot on the ticket on Thursday.
He chided the Bush administration for failing to take prompt action to shore up homeland defences after the September 11 attacks and swore to heed the conclusions of the national panel that probed the disaster.
Democratic officials had promised that the four-day convention would have minimal Bush-bashing and instead focus on telling the personal stories of Kerry and Edwards and highlighting their vision for the country.