Senator John Kerry, who lost to George W Bush in 2004, will not run for US president in 2008, he has said.
Mr Kerry disappointed many Democrats with his 2004 loss
The Massachusetts Democrat had been thought to be considering another run, and had kept campaign staff and a fundraising operation in place.
But he would have faced an uphill battle for his party's nomination, given the likely candidacies of Hillary Clinton and Barack Obama.
A visibly emotional Mr Kerry made the announcement on the Senate floor.
He said he had come close enough to victory in 2004 to be tempted to try again, but had concluded the time was not right.
He added that he would do "whatever he could" to ensure the next president inherited a more unified country.
He briefly choked up during his speech, which made reference to the Vietnam war, in which he served before becoming an anti-war protester.
He made his decision only in the past day, reports say.
Mr Kerry lost to Mr Bush by a margin of 51% to 48% in 2004, upsetting many Democrats who thought the president was beatable.
Senator Kerry has been actively campaigning for other Democratic candidates over the last year, raising money and making campaign appearances, leading many to believe he was planning to run again in 2008.
But he made what he called a "botched joke" in the waning days of November's mid-terms campaign, appearing to suggest that people who did not do well in school ended up as soldiers in Iraq.
He apologised and said he had been intending to criticise President Bush "for getting us stuck in Iraq", but the comment garnered such negative publicity that Mr Kerry kept a low profile in the wake of it.
At least a dozen candidates have begun making preparations for a White House bid in 2008.
The former First Lady, Senator Hillary Clinton, is among the highest-profile of them, and insiders say her fundraising ability will make it difficult for other candidates to compete.