Obama filed paperwork for a presidential exploratory committee. He first announced the decision on his Web site, www.barackobama.com.
Obama said on his Web site that he will spend a few weeks "exploring" with his committee, then announce his decision on running for president on Feb. 10 in Illinois, reports CBS News correspondent Bob Fuss.
When the world learns whether Obama will enter the race, his announcement will likely come in a historic city.
Several Obama supporters are being told it will happen in Springfield, Illinois, the hometown of President Lincoln. The day Obama picked, February tenth, is just two days before Lincoln's birthday.
"I certainly didn't expect to find myself in this position a year ago," Obama said in a video posting. "I've been struck by how hungry we all are for a different kind of politics. So I've spent some time thinking about how I could best advance the cause of change and progress that we so desperately need."
Obama, a 45-year-old a little more than two years into his Senate term, is the most inexperienced candidate considering a run for the Democratic nomination. He rose quickly to national prominence, beginning with his keynote speech at the 2004 Democratic National Convention and his election to the Senate that year, but is still an unknown quantity to many voters.
Two best-selling autobiographies — "The Audacity of Hope: Thoughts on Reclaiming the American Dream" and "Dreams from My Father: A Story of Race and Inheritance" — have helped fill in the gaps but have still only touched a fraction of the public.
Nonetheless, he ranks as a top contender. His appeal on the stump, his unique background, his opposition to the Iraq war and the fact that he is a fresh face set him apart in a competitive race that also is expected to include front-runner Sen. Hillary Rodham Clinton of New York.
Obama's advisers say he's not running as an alternative to anybody, but some Democrats call him the anti-Hillary, CBS News correspondent Gloria Borger reports. Polls show she's a lightening rod, and Obama is well-liked. But one problem for Obama is that he has few national security credentials beyond a couple of years on the Senate Foreign Relations Committee.
Other Democrats who have announced a campaign or exploratory committee are 2004 vice presidential nominee John Edwards, former Iowa Gov. Tom Vilsack, Connecticut Sen. Chris Dodd and Ohio Rep. Dennis Kucinich.
Both Obama and Clinton have hired senior aides while courting potential supporters in Iowa, New Hampshire and other key states.
Iowa's caucuses will kick off the nominating campaign in January 2008, to be followed by caucuses in Nevada and primaries in New Hampshire and South Carolina.
The moves by the two came as Dodd launched his campaign last week and New Mexico Gov. Bill Richardson prepared to make an announcement before the end of the month.
Obama and Clinton would clearly be the Democratic field's heavyweights — one vying to be the nation's first black president, the other the country's first female chief executive.
Obama's announcement Tuesday was comparatively low-key, banking on the hype building up to his decision to drive the buzz rather than a speech or high-profile media appearance. He was in Washington on Tuesday but did not plan any public appearances.