WASHINGTON — Sen. Barack Obama's decision to take the first formal step toward running for president caps an extraordinarily rapid rise in politics — and sets up a high-stakes competition for campaign money, staff and supporters for the Democratic nomination.

A state legislator just three years ago, Obama's announcement yesterday that he had formed a presidential exploratory committee establishes him as his party's most formidable rival to Sen. Hillary Rodham Clinton of New York. It also creates a face-off between the first strong black and female candidates for president, adding sizzle and a sense of historic significance to the competition for the party's nomination.

Hawai'i-born Obama, 45, is expected to announce a full-fledged candidacy on Feb. 10 in Springfield, Ill. He is gambling that voters will see his lack of national governing experience as an asset, not a liability, at a time when the electorate is seething with discontent with the Washington establishment.

"I am struck by how hungry we all are for a different kind of politics," Obama said in his announcement yesterday. "The decisions that have been made in Washington these past six years, and the problems that have been ignored, have put us in a precarious place."

In a sign of the importance of the Internet to political campaigning, Obama made his announcement not in a public appearance but in an e-mail statement and a video posted on his Web site.

"Our leaders in Washington seem incapable of working together in a practical, common-sense way," Obama said in a video posted on his Web site. "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. And that's what we have to change first."

DEMOCRATIC GOALS

Obama also said his campaign would emphasize traditional Democratic goals such as lowering healthcare costs, providing college tuition assistance and developing new energy sources. He only briefly mentioned the Iraq war, the issue that could well drive the 2008 election.

"We're still mired in a tragic and costly war that should never have been waged," Obama said.

In a brief interview on Capitol Hill, he said reaction to the possibility of his candidacy has been positive and added, "We wouldn't have gone forward this far if it hadn't been this positive."

His announcement steps up pressure on Clinton to formally launch her own campaign, a move that is expected soon. Howard Wolfson, a senior Clinton adviser, declined to comment on Obama but said, "Sen. Clinton has a strong case to make for her own candidacy and is going to have to make the best case for herself."

The combined star power and national fame of Clinton and Obama threaten to siphon off vast amounts of money and attention from lesser-known candidates in the crowded Democratic field.

Obama already is competing with Clinton for donors in California, the No. 1 source of money for Democrats. So far, he has been popular among major donors, despite his relatively short history of raising money in the state.

Clinton, who hired Los Angeles fundraiser Diane Hamwi on Sunday to oversee West Coast fundraising, has a huge head start.

As of last month, she had $14.4 million in campaign money in the bank; Obama had $756,000.

Obama was born in Honolulu, where his parents met while studying at the University of Hawai'i. His father was black and from Kenya; his mother, white and from Wichita, Kan.

Obama's parents divorced when he was 2 and his father returned to Kenya. His mother later married an Indonesian student, and the family moved to Jakarta. Obama returned to Hawai'i when he was 10 to live with his maternal grandparents.

He graduated from Columbia University and Harvard Law School, where he was the first black elected editor of the Harvard Law Review. Obama settled in Chicago, where he joined a law firm, helped local churches establish job-training programs and met his future wife, Michelle Robinson. They have two daughters, Malia and Sasha.

Obama lacks the deep roots other candidates have cultivated in states with early primaries and caucuses, such as Iowa. Still, he drew huge, enthusiastic crowds during his first-ever trip to New Hampshire, in December, even though most people knew little about him.

Mark Mellman, a Democratic pollster who is allied with no presidential candidate, said concern about Obama's inexperience may be eclipsed by a more powerful political force — voters' hunger for change, as evidenced in the 2006 midterm elections.

"People want something new and different, and there is nobody in this contest who can better lay claim to being new and different than Barack Obama," said Mellman.

Now, Obama faces heightened scrutiny that may spotlight less- flattering aspects of his past. In his first book, Obama acknowledged that he experimented with cocaine and marijuana in his youth.

VOTING RECORD

His record will also come under closer scrutiny. He has a traditionally liberal voting history, supporting gay rights, abortion rights and gun control. The only vote in 2006 where he opposed the liberal Americans for Democratic Action was in supporting a free-trade agreement with Oman.

Obama has been a harsher critic of the war in Iraq than Clinton. Although he was not in the U.S. Senate in 2002, when Clinton voted to authorize the invasion of Iraq, Obama at the time spoke out against the war.

Now, both Obama and Clinton are opposing President Bush's proposal to increase troop levels in Iraq, but Clinton has been more low-key and moderate in her tone.

The Los Angeles Times, Washington Post and Associated Press contributed to this report.