Senator Barack Obama is taking the first step towards a White House run, raising the possibility the US will get its first black president.
Mr Obama, a Democrat, said on his website he had formed an exploratory committee, which would allow him to raise money and hire staff for the run.
He is one of his party's rising stars, having electrified the 2004 Democratic convention with a powerful speech.
He opposed the war in Iraq and has a solidly liberal Senate voting record.
In a video announcement posted on his website, he said Americans wanted to move beyond partisan politics to find common-sense solutions to problems.
"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," he said.
"We have to change our politics, and come together around our common interests and concerns as Americans." [Read more]