Have third parties ever made a difference in presidential elections?

No third party has ever captured the White House because it requires at least 270 electoral votes to do so, and a candidate must win a majority of the popular vote in any given state to collect that state's electoral votes. That does not mean that third party candidates may not have influenced the outcome of the election. The election of 1912 went to the democratic candidate, Woodrow Wilson, because Progressive Party candidate Theodore Roosevelt drained votes away from the Republican incumbent Howard Taft. In more recent times, H. Ross Perot may have contributed to the Bill Clinton's defeat of Republican incumbent George H. W. Bush when Reform Party voters cut into the Republican base. The 2000 election led to the selection of a "minority president" when Green Party candidate Ralph Nader shaved enough votes from the democratic front runner, vice president Al Gore, to lead to a contested popular vote in Florida and other states. The 5-4 supreme court decision to stop the counting of votes in Florida gave George Bush Florida's 26 electoral votes, even though Gore won the popular vote nationwide. Check out the results of elections in the popular and electoral votes at http://www.270towin.com/.