2008 presidential campaign

Clinton had been preparing for a potential candidacy for U.S. president since at least early 2003. On January 20, 2007, she announced via her website the formation of a presidential exploratory committee for the United States presidential election of 2008, stating: "I'm in and I'm in to win." No woman had ever been nominated by a major party for the presidency, and no first lady had ever run for president. When Bill Clinton became president in 1993, a blind trust was established; in April 2007, the Clintons liquidated the blind trust to avoid the possibility of ethical conflicts or political embarrassments as Hillary undertook her presidential race. Later disclosure statements revealed the couple's worth was now upwards of $50 million. They had earned over $100 million since 2000—most of it coming from Bill's books, speaking engagements and other activities.

Throughout the first half of 2007, Clinton led candidates competing for the Democratic presidential nomination in opinion polls for the election. Senator Barack Obama of Illinois and former senator John Edwards of North Carolina were her strongest competitors. The biggest threat to her campaign was her past support of the Iraq War, which Obama had opposed from the beginning. Clinton and Obama both set records for early fundraising, swapping the money lead each quarter. At the end of October, Clinton fared poorly in her debate performance against Obama, Edwards, and her other opponents. Obama's message of change began to resonate with the Democratic electorate better than Clinton's message of experience.

In the first vote of 2008, she placed third in the January 3 Iowa Democratic caucus behind Obama and Edwards. Obama gained ground in national polling in the next few days, with all polls predicting a victory for him in the New Hampshire primary. Clinton gained a surprise win there on January 8, narrowly defeating Obama. It was the first time a woman had won a major American party's presidential primary for the purposes of delegate selection. Explanations for Clinton's New Hampshire comeback varied but often centered on her being seen more sympathetically, especially by women, after her eyes welled with tears and her voice broke while responding to a voter's question the day before the election.

The nature of the contest fractured in the next few days. Several remarks by Bill Clinton and other surrogates, and a remark by Hillary Clinton concerning Martin Luther King Jr. and Lyndon B. Johnson,i were perceived by many as, accidentally or intentionally, limiting Obama as a racially oriented candidate or otherwise denying the post-racial significance and accomplishments of his campaign. Despite attempts by both Hillary and Obama to downplay the issue, Democratic voting became more polarized as a result, with Clinton losing much of her support among African Americans. She lost by a two-to-one margin to Obama in the January 26, South Carolina primary, setting up, with Edwards soon dropping out, an intense two-person contest for the twenty-two February 5 Super Tuesday states. Bill Clinton had made more statements attracting criticism for their perceived racial implications late in the South Carolina campaign, and his role was seen as damaging enough to her that a wave of supporters within and outside of the campaign said the former president "needs to stop". The South Carolina campaign had done lasting damage to Clinton, eroding her support among the Democratic establishment and leading to the prized endorsement of Obama by Ted Kennedy.

On Super Tuesday, Clinton won the largest states, such as California, New York, New Jersey and Massachusetts, while Obama won more states; they almost evenly split the total popular vote. But Obama was gaining more pledged delegates for his share of the popular vote due to better exploitation of the Democratic proportional allocation rules.

The Clinton campaign had counted on winning the nomination by Super Tuesday and was unprepared financially and logistically for a prolonged effort; lagging in Internet fundraising as Clinton began loaning money to her campaign. There was continuous turmoil within the campaign staff, and she made several top-level personnel changes. Obama won the next eleven February contests across the country, often by large margins and took a significant pledged delegate lead over Clinton. On March 4, Clinton broke the string of losses by winning in Ohio among other places, where her criticism of NAFTA, a major legacy of her husband's presidency, helped in a state where the trade agreement was unpopular. Throughout the campaign, Obama dominated caucuses, for which the Clinton campaign largely ignored and failed to prepare. Obama did well in primaries where African Americans or younger, college-educated, or more affluent voters were heavily represented; Clinton did well in primaries where Hispanics or older, non-college-educated, or working-class white voters predominated. Behind in delegates, Clinton's best hope of winning the nomination came in persuading uncommitted, party-appointed superdelegates.

Following the final primaries on June 3, 2008, Obama had gained enough delegates to become the presumptive nominee. In a speech before her supporters on June 7, Clinton ended her campaign and endorsed Obama. By campaign's end, Clinton had won 1,640 pledged delegates to Obama's 1,763; at the time of the clinching, Clinton had 286 superdelegates to Obama's 395, with those numbers widening to 256 versus 438 once Obama was acknowledged the winner. Clinton and Obama each received over 17 million votes during the nomination processj with both breaking the previous record. Clinton was the first woman to run in the primary or caucus of every state and she eclipsed, by a very wide margin, Congresswoman Shirley Chisholm's 1972 marks for most votes garnered and delegates won by a woman. Clinton gave a passionate speech supporting Obama at the 2008 Democratic National Convention and campaigned frequently for him in fall 2008, which concluded with his victory over McCain in the general election on November 4. Clinton's campaign ended up severely in debt; she owed millions of dollars to outside vendors and wrote off the $13 million that she lent it herself. The debt was eventually paid off by the beginning of 2013.