US senator John Kerry, the Democrat who lost the 2004 election to incumbent President George W Bush, said on Wednesday he would not make a second run in 2008.
Kerry, 63, had been considered a possible presidential candidate. Choking up with emotion, he said he would instead seek re-election to the senate next year to help lead opposition to Bush and the war in Iraq.
"There are powerful reasons to continue that fight now. But I've concluded this isn't the time for me to mount a presidential campaign," he said during a speech in the senate.
The senator from the north-eastern state of Massachusetts has rallied little support in his centre-left party, where former US first lady Hillary Clinton is the front-runner in an already crowded field of 2008 presidential hopefuls.
Recent polls on possible Democratic contenders have also placed Kerry behind Barack Obama, a charismatic young African-American senator, as well as Kerry's 2004 running mate John Edwards and former vice president Al Gore.
Kerry lost the 2004 election after a bitter campaign in which Bush and his Republican supporters accused him of wavering on the war in Iraq and attacked his record of military service in the Vietnam War, where he fought as a volunteer before becoming disillusioned.
In bowing out of the 2008 race Wednesday, Kerry choked back tears as he condemned "a policy in Iraq that threatens all that I've cared about and fought for since I came home from Vietnam".
Kerry, who has served in the senate since 1985, vowed to remain an outspoken opponent of the US military involvement in Iraq.
"I intend to speak the truth as I find it without regard to political correctness or partisan advantage," he said.