He was elected to the Illinois Senate in 1996. Four years later, he made an unsuccessful run for the U.S. House of Representatives. Obama won reelection to the state senate in 2002, running unopposed. As early as 2002, he was a critic of the proposed Iraq War, declaring in a television interview that he would have voted against the Iraq Resolution. In 2004 he ran for an open seat in the U.S. Senate. Midway through the campaign, Obama delivered the keynote address at the 2004 Democratic National Convention and became a nationally known political figure. He was elected to the U.S. Senate in November 2004 with a landslide 70% of the vote.

Obama formally announced his candidacy for the 2008 presidential election in Springfield, Illinois, on February 10, 2007. Recent opinion polls rank him as the second most popular choice among Democratic voters for their party's nomination, after Sen. Hillary Rodham Clinton (D-NY).

Barack Obama was born in Honolulu, Hawaii to Barack Hussein Obama, Sr. (born in Alego, a village in Nyanza Province, Kenya) and Ann Dunham (born in Wichita, Kansas). His parents met while both were attending the East-West Center of the University of Hawaii at Manoa, where his father was enrolled as a foreign student.

When Obama was two years old, his parents separated and later divorced; his father went to Harvard University to pursue Ph.D. studies, eventually returning to Kenya. His mother married Lolo Soetoro, an Indonesian foreign student, with whom she had one daughter. The family moved to Jakarta in 1967, where Obama attended local schools from ages 6 to 10 He then returned to Honolulu to live with his maternal grandparents, attending Punahou School from 5th through 12th grade and graduating from there in 1979. His father died in a car accident in Kenya when Obama was 21 years old. Obama's mother died of cancer a few months after the publication of his 1995 memoir, Dreams from My Father.

In Dreams from My Father, Obama describes his experiences growing up in his mother's white, middle class family. His knowledge about his absent black Kenyan father came mainly through family stories and photographs. Of his early childhood, Obama wrote: "That my father looked nothing like the people around me — that he was black as pitch, my mother white as milk — barely registered in my mind." As a young adult, he struggled to reconcile social perceptions of his multiracial heritage. Obama writes about using marijuana and cocaine during his teenage years to "push questions of who I was out of my mind."

After high school, Obama studied for two years at Occidental College and then transferred to Columbia University, where he majored in political science with a specialization in international relations. After receiving his B.A. degree in 1983, Obama worked for one year at Business International Corporation. In 1985, he moved to Chicago to direct a non-profit project assisting local churches to organize job training programs for residents of poor neighborhoods.

Obama entered Harvard Law School in 1988. In February 1990, the New York Times reported his election as the Harvard Law Review's "first black president in its 104-year history." He obtained his J.D. degree magna cum laude from Harvard in 1991. On returning to Chicago, Obama directed a voter registration drive, then worked for the civil rights law firm Miner, Barnhill & Galland, and taught constitutional law at the University of Chicago Law School from 1993 until his election to the U.S. Senate in 2004.

In 1996, Obama was elected to the Illinois State Senate from the state's 13th District in the south-side Chicago neighborhood of Hyde Park. In January 2003, when Democrats regained control of the chamber, he was named chairman of the Senate Health and Human Services Committee. Among his legislative initiatives, Obama helped to author an Illinois Earned Income Tax Credit that provided benefits to lower-income families, worked for legislation that would support residents who could not afford health insurance, and helped pass bills to increase funding for AIDS prevention and care programs.

In 2000, Obama made an unsuccessful Democratic primary run for the U.S. House of Representatives seat held by four-term incumbent candidate Bobby Rush. Rush, a former Black Panther and community activist, said that Obama had not "been around the 1st Congressional District long enough to really see what's going on." Rush received 61% of the vote to Obama's 30%. After the loss, Obama focused his efforts on the state Senate, authoring a law requiring police to videotape interrogations for crimes punishable by the death penalty and supporting legislation that required insurance companies to cover routine mammograms. He ran unopposed in 2002.

Reviewing Obama's career in the Illinois Senate, a February 2007 article in the Washington Post noted his ability to work effectively with both Democrats and Republicans, and to build bipartisan coalitions. In his subsequent campaign for the U.S. Senate, Obama won the endorsement of the Illinois Fraternal Order of Police, whose officials cited his "longtime support of gun control measures and his willingness to negotiate compromises," despite his support for some bills that the police union had opposed.

On February 10, 2007, in Springfield, Illinois, Obama announced his candidacy for the 2008 U.S. presidential election. He said:

It was here, in Springfield, where North, South, East and West come together that I was reminded of the essential decency of the American people–where I came to believe that through this decency, we can build a more hopeful America. 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.

The announcement followed months of speculation on whether Obama would run in 2008. Speculation intensified in October 2006 when Obama first said he had "thought about the possibility" of running for president, departing from earlier statements that he intended to serve out his six-year Senate term through 2010. Following Obama's statement, opinion polling organizations added his name to surveyed lists of Democratic candidates. The first such poll, taken in November 2006, ranked Obama in second place with 17% support among Democrats after Sen. Hillary Clinton (D-NY) who placed first with 28% of the responses. Analysts are expected to note whether Obama's poll numbers prove accurate, or become subject to the so-called "Bradley effect".

Through the fall of 2006, Obama spoke at political events across the country in support of Democratic candidates for the midterm elections. In September 2006, he was the featured speaker at Iowa Senator Tom Harkin's annual steak fry, an event traditionally attended by presidential hopefuls in the lead-up to the Iowa caucus. In December 2006, Obama spoke at a New Hampshire event celebrating Democratic Party midterm election victories in the first-in-the-nation U.S. presidential primary state. Addressing a meeting of the Democratic National Committee one week before announcing his candidacy, Obama called on Democrats to steer clear of negative campaigning:

This is not a game. This can't be about who digs up more skeletons on who, who makes the fewest slip-ups on the campaign trail. We owe it to the American people to do more than that. We owe them an election where voters are inspired–where they believe that we might be able to do things that we haven't done before. We don't want another election where voters are simply holding their noses and feel like they're choosing the lesser of two evils. So we've got to rise up out of the cynicism that's become so pervasive and ask the people all across America to start believing again.