New York junior Senator and former First Lady Hillary Rodham Clinton had expressed interest in the 2008 United States presidential race since at least October 2002, drawing media speculation on whether she would become a candidate. No woman has ever won the nomination of a major party in the history of U.S. presidential elections.

On January 20, 2007, she announced that she was forming an exploratory committee and filed paperwork with the Federal Election Commission. Subsequently she began heavy fundraising and campaigning activities. For several months Clinton led opinion polls among Democratic candidates by substantial margins, until Senator Barack Obama pulled close to or even with her. Clinton then regained her lead, winning many polls by double digits; by autumn 2007 she was leading all other Democratic candidates by wide margins in national polls. She placed third in the Iowa caucus to Barack Obama and John Edwards, and trailed badly in polls shortly thereafter in New Hampshire, before staging a sudden and dramatic comeback and finishing first in the New Hampshire primary. She went on to win in Nevada, but lost by a heavy margin in South Carolina. On Super Tuesday, Clinton won delegate-rich states such as California and New York, while Obama won more states; the two gained a nearly equal number of delegates and a nearly equal share of the total popular vote. Clinton then lost the next eleven caucuses and primaries to Obama, and lost the overall delegate lead to him for the first time. On March 4, his consecutive wins increased to twelve when Vermont was called in his favor but after an increasingly aggressive round of campaigning, Clinton broke the string of losses with wins in the Rhode Island, Ohio, and Texas primaries.