Barack Obama evoked the memory of Abraham Lincoln as he officially began his marathon race to become the first black president of the United states.

"We can build a more hopeful America," the freshman U.S. Democratic senator told supporters Saturday in Springfield, Ill..

"And that is why, in the shadow of the Old State Capitol, where Lincoln once called on a house divided to stand together, where common hopes and common dreams still live, I stand before you today to announce my candidacy for President of the United States of America."

Obama himself began his career in elected politics as an Illinois state senator.

During his speech, Obama reminded supporters he was opposed to the war in Iraq from the beginning and said he has a plan to bring U.S. combat troops home by March 2008.

He also spoke about the need to act quickly on climate change and the environment, and suggested people have been shut out by the current administration and want a government that is more representative of their beliefs.

"You came here because you believe in what this country can be," Obama said.

"In the face of war you believe there can be peace. In the face of despair, you believe there can be hope. In the face of a politics that shut you out, that told you to settle, that has divided us for too long, you believed we can be one people reaching out for what's possible, building that more perfect union. That's the journey we're on today."

Still in his first term as a U.S. senator, Obama acknowledged his declaration could seem presumptuous, saying: "I know I haven't spent a lot of time learning the ways of Washington, but I've been there long enough to know the ways of Washington must change."

He said his political career -- and his decision to enter the race -- has been motivated by "a single, simple, powerful idea that I might play a small part in building a better America."

Obama spent much of his speech talking about the journey that brought him to politics.

He said he first came to Chicago 20 years ago to work as a community organizer for church groups with a salary of US$13,000 a year, where he said his Christian faith was strengthened.

Obama said he spent time in some of the city's poorest neighbourhoods during that time, gaining an education that money couldn't buy.

After three years he said he went to law school "to learn how the law should work for those in need," and eventually became a civil rights lawyer where he learned that "our cherished rights of liberty and equality depend on the active participation of an awakened electorate."

Obama, 45, then entered politics. He served eight years in the Illinois Legislature in Springfield.

Life story

More than anything he is selling his own personal life story," Mark Plotkin, a U.S. political analyst, told Newsnet.

"He's an immensely likable person. The story he tells about his own journey is terrific in the all-American experience.

"He's wrapping himself in the Abraham Lincoln mold -- a person that can bring the country together. I thought the speech was pretty short on specification and pretty lofty on grand themes."

Plotkin said Obama's greatest challenge now lies ahead. He must raise between US$50 and $75 million, and convince voters across America that he is a worthy candidate.

"And I think people are going to ask for more than just the personal story. Probably one major issue that he's very strong on is that he can point to the difference between himself and John Edwards and Hillary Clinton -- that he was against the war from the beginning. And that will draw a lot of support."

Obama gained national recognition with the publication of two bestselling books -- Dreams From My Father and The Audacity of Hope -- and by delivering an electrifying keynote address at the 2004 Democratic National Convention.

Last fall Obama said he was considering running after receiving so much encouragement, despite having said earlier he wasn't planning to run.

In 2004, he said: "People are talking about (my being a candidate). Well that's silly talk. Ask my wife, she'll tell you I need to learn to put my socks in the hamper."

He's gotten some powerful support from Chicago TV host Oprah Winfrey and celebrity treatment from People magazine.

Obama, the son of a white mother from Kansas and a black father from Kenya, is widely seen as the chief rival of New York Senator Hillary Rodham Clinton.

The Democratic Party will choose its candidate at a convention in Denver in late August 2008.

However, early polls put Clinton, wife of former U.S. President Bill Clinton, as the front-runner by a substantial margin.

There are other, more senior politicians in the race, such as Delaware Sen. Joe Biden, former New Mexico Gov. Bill Richardson and former North Carolina senator John Edwards, the Democrats' vice-presidential nominee in 2004.

However, Obama has gone right to work. He travelled to Cedar Rapids, Iowa for a speech. The Iowa caucuses will be the first in early 2008 to pick which Democratic candidate they support.

Obama will also travel to New Hampshire, another early primaries state, and his hometown of Chicago in the next few days.

With a report from CTV's Tom Walters and files from the Associated Press