Barack Obama, America’s fastest rising political star, on Tuesday beat Hillary Clinton to the punch when he announced he was taking the first legal step towards declaring his 2008 presidential candidacy.

The Democratic senator, who first came to national prominence when he gave a widely-admired speech at John Kerry’s 2004 presidential convention in Boston, said he was responding to the hunger of Americans for “a different kind of politics”.

Born of mixed parentage to a Kenyan father and a white American mother, the 45-year-old first term senator from Illinois is unique among the prospective Democratic hopefuls in having publicly opposed George W. Bush’s 2003 now deeply-unpopular invasion of Iraq.

But Democratic consultants say that it is Mr Obama’s promise of bringing a new kind of politics to America that has propelled him to such meteoric heights among the party’s rank and file.

Mr Obama, whose book, The Audacity of Hope, has been at the top of the bestseller lists since it was launched last October, has been endorsed by Oprah Winfrey, the influential talk show host.

“Challenging as they are, it’s not the magnitude of our problems that concerns me the most,” said Mr Obama on Tuesday in a video posting on his campaign website. “It’s the smallness of our politics….Politics has become so bitter and partisan, so gummed up by money and influence, that we can’t tackle the big problems that demand solutions.”

Tuesday’s announcement is likely to intensify pressure on Hillary Clinton, the former first lady and senator for New York, to clarify her presidential aspirations. Like Mr Obama, Mrs Clinton has already hired dozens of campaign consultants in what is widely seen as a preparation for a bid.

Unlike Mr Obama, who freely admitted in an earlier book to having taken marijuana and cocaine in his youth, Mrs Clinton has based her political career on systematic caution, always consulting as widely as possible before articulating highly calibrated policy announcements. Democratic consultants say that Mrs Clinton’s biggest challenge will be to overcome the hostility of the party’s activist base to the fact that she overtly supported ? and voted for - Mr Bush’s war in Iraq.

The formal entrance of Mr Obama into the race will intensify what is already America’s most open and competitive presidential race in years. Candidates are scheduled to hold their first public debates in New Hampshire this April ? a full nine months before they submit themselves to party primary elections.

“This is the most open presidential contest in living memory,” said Simon Rosenberg, director of the New Democratic Network, a centrist group. “The fact that we have such a weak White House that will bequeath a troubling legacy to the next president is also driving this early campaign.”

Mr Obama is the sixth Democrat formally to declare his candidacy or set up an exploratory committee. Others include John Edwards, the former vice-presidential candidate, and Christopher Dodd, the Connecticut senator.

Mr Obama, who says he will make a final announcement on his candidacy on February 10, has a strong liberal voting record in the Senate but has steered clear of detailed policy platforms on issues such as healthcare, trade and foreign policy other than Iraq.

Among the Republicans, John McCain, the failed 2000 presidential candidate, and Mitt Romney, the former governor of Massachusetts, are considered to be the frontrunners. But both parties are expected to have multiple candidates. Polls put Mr Obama a close second to Mrs Clinton among registered Democrats.