SPRINGFIELD, Ill. — Illinois Sen. Barack Obama formally announced his presidential candidacy yesterday with an invocation of Abraham Lincoln and a call to end the war in Iraq and what he called "the smallness of our politics" that perpetuates so many of the problems the nation faces.

In a speech that recalled his remarks to the 2004 Democratic National Convention — the address that launched his political rocket — Obama sought to set himself apart not just from the rest of the Democratic field, but the Washington, D.C., establishment he recently joined.

He ticked off a number of challenges before the country: "A war with no end, a dependence on oil that threatens our future, schools where too many children aren't learning, and families struggling paycheck to paycheck despite working as hard as they can. We know the challenges. We've heard them. We've talked about them for years. What's stopped us from meeting these challenges is not the absence of sound policies and sensible plans."

Rather, he suggested, the problems stem from something larger:

"The failure of leadership, the smallness of our politics — the ease with which we're distracted by the petty and trivial, our chronic avoidance of tough decisions, our preference for scoring cheap political points instead of rolling up our sleeves and building a working consensus to tackle big problems."

Obama delivered his remarks before the Old State Capitol on a biting cold morning. Thousands of people braved temperatures in the single digits to be part of a day tinged with history; whatever the outcome of the 2008 campaign, Obama already is regarded as the most formidable black candidate to seek the White House. If elected, he would become the nation's first black president.

The site chosen for his announcement was rich in symbolism. Lincoln served as a state representative in the Old State Capitol and, during an 1858 debate with Stephen Douglas, delivered his famous anti-slavery speech quoting from the Bible and stating "a house divided against itself cannot stand."

Obama cited those remarks and invoked Lincoln at several other points in his speech.

"Through his will and his words, he moved a nation and helped free a people," Obama said. "It is because of the millions who rallied to his cause that we are no longer divided, North and South, slave and free. It is because men and women of every race, from every walk of life, continued to march for freedom long after Lincoln was laid to rest that today we have the chance to face the challenges of this millennium together, as one people — as Americans."

Obama, 45, the son of a black Kenyan father and white American mother, recounted his work as a Chicago community activist and cited the seven years he spent in the Illinois Senate before going to Washington, D.C., in 2005. He said Springfield was the place where he learned the kind of politics he hopes to practice as president.

"It was here we learned to disagree without being disagreeable," Obama said, "that it's possible to compromise so long as you know those principles that can never be compromised; and that so long as we're willing to listen to each other, we can assume the best in people instead of the worst."

He cited his limited time in Washington, just two years in the U.S. Senate, as a plus rather than a detriment. "I've been there long enough to know that the ways of Washington must change," he said.

On the war in Iraq, Obama offered one of the few explicit policy statements of his speech:

"It's time to admit that no amount of American lives can resolve the political disagreement that lies at the heart of someone else's civil war," Obama said. "That's why I have a plan that will bring our combat troops home by March of 2008. Letting the Iraqis know that we will not be there forever is our last, best hope to pressure the Sunni and Shia to come to the table and find peace."

Obama enters the race as one of three hopefuls generally regarded as the top tier of candidates on the Democratic side. New York Sen. Hillary Rodham Clinton widely is seen as the front-runner, with former North Carolina Sen. John Edwards filling out the top three.

After his announcement, Obama headed into a weekend of campaigning in Iowa, the state that begins the nominating process with its precinct caucuses. Obama plans to travel tomorrow to New Hampshire, the state that holds the first primary.