Legislative career: U.S. Senate campaign

In May 2002, Obama commissioned a poll to assess his prospects in a 2004 U.S. Senate race; he created a campaign committee, began raising funds and lined up political media consultant David Axelrod by August 2002, and formally announced his candidacy in January 2003.

Obama was an early opponent of the George W. Bush administration's 2003 invasion of Iraq. On October 2, 2002, the day President Bush and Congress agreed on the joint resolution authorizing the Iraq War, Obama addressed the first high-profile Chicago anti-Iraq War rally, and spoke out against the war. He addressed another anti-war rally in March 2003 and told the crowd that "it's not too late" to stop the war.

Decisions by Republican incumbent Peter Fitzgerald and his Democratic predecessor Carol Moseley Braun not to contest the race resulted in wide-open Democratic and Republican primary contests involving fifteen candidates.

In the March 2004 primary election, Obama won in an unexpected landslide—which overnight made him a rising star within the national Democratic Party, started speculation about a presidential future, and led to the reissue of his memoir, Dreams from My Father.

In July 2004, Obama delivered the keynote address at the 2004 Democratic National Convention in Boston, Massachusetts, and it was seen by 9.1 million viewers. His speech was well received and elevated his status within the Democratic Party.

Obama's expected opponent in the general election, Republican primary winner Jack Ryan, withdrew from the race in June 2004. Six weeks later, Alan Keyes accepted the Illinois Republican Party's nomination to replace Ryan. In the November 2004 general election, Obama won with 70% of the vote.