Bill Richardson, governor of the US state of New Mexico, has become the latest Democrat to declare an interest in the 2008 presidential election.
Bill Richardson served as ambassador to the UN
He has announced that he is setting up a presidential exploratory committee, the first step in the race.
Mr Richardson would become the first Hispanic president, if elected.
Former First Lady Hillary Clinton and African-American Senator Barack Obama are also running in what correspondents say will be a highly diverse contest.
Mr Richardson, 59, has served in a number of senior government posts. He was energy secretary, and the US ambassador to the United Nations during Bill Clinton's presidency.
He is also a former congressman, and would hope to mobilise the considerable Hispanic vote in the US.
However, he does not have the level of support enjoyed by Senator Clinton, Senator Obama or John Edwards, who was John Kerry's running mate for the Democrats in the 2004 election.
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"I am taking this step because we have to repair the damage that's been done to our country over the last six years," said Mr Richardson, in a statement announcing his move.
"Our reputation in the world is diminished, our economy has languished, and civility and common decency in government has perished."
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His move comes a day after Senator Hillary Clinton announced her candidacy, saying: "I'm in - and I'm in to win."
Senator Clinton said she planned to answer questions in web chats on three consecutive nights, starting on Monday.
She is currently serving as a senator for New York, having won a second term by a landslide last November.
Correspondents say her unbeatable name recognition and unmatched fundraising ability make her a clear front runner for the Democrats.