"I am not going to stand by and watch thousands more of our brave, young men and women killed in Iraq," Kucinich said to applause from a crowd gathered at City Hall. "We Democrats were put back in power to bring some sanity back to our nation.
"We were expected to do what we said we were going to do — get out of Iraq."
Kucinich is a six-term, liberal congressman from Cleveland whose presidential candidacy in 2004 made headlines more for his bachelorhood than his policies. This time around Kucinich has a wife.
Kucinich, 60, said he was inspired to run because he disagrees with the way some of his fellow Democrats are handling the war, including approval of a proposal to spend $160 billion more on the conflict.
Kucinich joins Iowa Gov. Tom Vilsack, a fellow Democrat, in declaring his candidacy for the presidential nomination.
New York Sen. Hillary Rodham Clinton is considered the party's front-runner, closely followed by Illinois Sen. Barack Obama. Neither has announced runs.
In 2004, Kucinich posted single digits in most primary elections, including his home state, yet stayed in the race.
He celebrated his bachelorhood on the campaign trail, telling New Hampshire audiences that he was seeking a mate. Women vied for a date with him during an online contest, but nothing romantic evolved from Kucinich's breakfast date with the winner. It did earn him appearances on late-night comedy talk shows.
Last year he married Elizabeth Harper. He won re-election to his House seat in 2006 with 66 percent of the vote, basing his campaign on job creation and criticizing rising gas prices. He also was an outspoken critic of his own party, saying Democrats have lost their soul by moving away from liberal ideals.
Kucinich was elected mayor of Cleveland at age 31, the youngest leader of a major American city. He also became the mayor of the first city since the Great Depression to go into default.