Bob Barr was the 2008 Libertarian candidate for President of the United States. Though Barr was a conservative Republican during his eight years in the US House of Representatives, in 2003 he expressed frustration with the power-gathering of the Republican President George W. Bush aided by the Republican Congress, and Barr vocally adopted a number of Libertarian positions, including opposition to the Patriot Act, the War in Iraq, criminalization of medical marijuana. Barr also reversed his position on the Defense of Marriage Act, now wishing to eliminate any federal government authority to define marriage. Barr reversed his previous opposition to Wiccans and other religious minorities having services on military bases, a position that George W. Bush had also expressed in 1999.

In other areas, Barr had always supported the Libertarian position, such as supporting strong Second Amendment rights, opposing high taxes, high government spending, and bureaucratic regulation. Barr remained somewhat at odds with Libertarians on the issue of abortion, which Barr still strongly opposes, although he would leave it entirely to the states to determine whether they would permit or prohibit the practice. The Libertarian platform has long advocated an absence of regulation of the practice, even at the state level.

As of August 2008, Barr was polling as much as 10% in certain states such as New Hampshire, Oklahoma, and Nevada, and has been deemed by some as a potential spoiler who could cost John McCain the election. Barr's campaign manager, Russ Verney, chalked "spoiler" talk up to the McCain campaign wanting to have a ready excuse for losing. Barr followed a strategy that would not require him to win outright in the Electoral College. He relied instead on McCain and Barack Obama to run a fairly even race, giving Barr a possibility of hanging the Electoral College by winning just a handful of Libertarian-leaning states such as New Hampshire, Wyoming, Montana, and Alaska, possibly along with Barr's current home state of Georgia, and his birth state of Iowa.

Barr did not win any states, getting about a half-million votes as Barack Obama won the election in a landslide, registering the biggest victory by a Democrat in over forty years. But in two states, North Carolina and Indiana, the vote for Barr was greater than the difference between Obama and McCain, meaning that if Barr voters had instead gone with McCain, those states would have been won by McCain.