Chris Lehane's departure comes amid speculation of a wider shake-up in the Kerry campaign, which has been torn by internal fights and a lack of public support from the candidate.
Kerry, a Massachusetts senator once considered the leading contender in a nine-person field, has seen his campaign eclipsed by former Vermont Gov. Howard Dean.
"John Kerry is a great American," Lehane said in a statement confirming his resignation. "He has assembled a great team to take on George W. Bush and I wish him the best of luck as the campaign goes forward."
Lehane was a key adviser and spokesman for the campaign, though he was not on the payroll. That move was planned later this fall. He resigned last week.
Campaign officials, speaking on condition of anonymity, said Lehane told them he was leaving over philosophical differences with Kerry. They said Lehane, who was Al Gore's press secretary in the 2000 race and worked for President Clinton, was among a cadre of Kerry aides who believed that Kerry ran too cautiously against the threat posed by Dean.
Campaign strategist Bob Shrum and others urged Kerry to remain above the fray in an attempt to look presidential. Kerry avoided confrontation with Dean in the first two debates, but his rhetoric on the campaign trail has become more critical of the former governor.
The Los Angeles Times, citing campaign sources, reported that Lehane was unhappy with the speech Kerry delivered in South Carolina last week formally launching his presidential campaign.
The newspaper said Lehane believed Kerry should have presented a broader message instead of continuing to emphasize his background as a decorated Vietnam veteran.
Dean leads Kerry in the latest polls in New Hampshire, an early voting state that neither candidate can afford to lose. His front-runner status lost, Kerry recently dropped out of contention for at least one key union endorsement and is scrambling to shore up support in Congress and among party donors.
Though Kerry has insisted he's satisfied with his team, his less-than-firm denials of a shake-up have fueled rumors and created angst among his staff.
Early this month, as he formally began his campaign, Kerry told reporters he "reserved the right" to make changes and gave a mixed assessment of his staff's performance. Trying to quell talk of a staff purge, he issued a statement saying there would be "no changes."
The statement, drafted with Lehane's assistance, was meant to be the last word, but Kerry has backpedaled from it. "Those weren't precisely my words. They were the words of a press release sent out," Kerry told The Boston Globe in a story published Sunday.
Several campaign sources said at the time that Kerry read and approved the document.
Kerry also told the Globe he would add people to "plug holes" but declined to be specific.
Campaign officials said there are long-standing plans to broaden Kerry's team, particularly with allies from Massachusetts with presidential campaign experience, but that's not necessarily an indictment of his current team, headed by campaign manager Jim Jordan.
Jordan said of Lehane: "He would've been a hugely valuable addition to the staff, and we're all disappointed. We'll plug the hole soon enough."