The phenomenon should not be confused with that of faithless electors, who pledge their vote to a candidate before the election but ultimately vote for someone else.
When the United States Constitution was written, the Founding Fathers intended the Electoral College to be a truly deliberative body whose members would choose a President based on their own preferences. They also left the method for selecting the electors for each state to the discretion of that state's legislature. Thus, the Constitution places no restriction on the behavior of the electors, and assumes that each is an independent agent.
While the electors in the first few U.S. presidential elections may have cast their votes along these lines, U.S. politics very quickly became dominated by strong political party organizations, and by the 1830s most states chose their electors by popular vote. As a result, the electors who appeared on ballots were nominated by the state chapters of national parties with the understanding that they would cast their votes for their party's candidate if elected. This became such a given in Presidential elections that most states eventually stopped listing the names of the electors on ballots, listing the candidate to whom those electors were pledged instead.
After the American Civil War and Reconstruction, the Democratic Party gained an almost unbreakable dominance in the Southern United States, and the Republicans, associated with Abraham Lincoln and the Union cause, were correspondingly unelectable. While the leaders of the Democratic Party in the South were in many aspects conservative, especially in regard to segregation and civil rights for African Americans, the nationwide Democratic party became increasingly liberal in early 20th century, a movement that accelerated with the election of Franklin D. Roosevelt. In several mid-20th century elections, Democrats put slates of unpledged electors on the ballots in several Southern states; in some cases they ran in opposition to electors pledged to the nationwide Democratic candidate, and in others they were the only Democratic electors that appeared on the ballot. The goal was to have electors who could act as kingmakers in a close election, extracting concessions that would favor conservative Southern Democrats in exchange for their votes.
The first modern slates of unpledged electors were fielded in the 1944 election as a protest against FDR's New Deal and support for desegregation. In Texas, a splinter group of Democrats known as the Texas Regulars fielded a slate of electors not pledged to any candidate; a similar slate was on the ballot in South Carolina. Neither group met much success.
In 1956, unpledged slates were on the ballot in Alabama (20,150 votes, 4.1% of the vote), Louisiana (44,520 votes, 7.2% of the vote and they won four parishes), Mississippi (42,266 votes, 17.3% of the vote and they won seven counties) and South Carolina (88,509 votes, 29.5% of the vote and 21 counties).
The 1960 election was the only election that saw unpledged electors actually elected to the electoral college. In that year, a slate of eight unpledged electors in Mississippi won a plurality of the vote there (116,248 votes, or 39% of the total). In Alabama, the state Democratic party nominated a mixed slate of electors, five of whom were pledged to Democratic nominee John F. Kennedy and six of whom were unpledged; this slate won the election in that state. Louisiana's popular vote went to a slate of electors pledged to Kennedy, but a slate of unpledged electors on the ballot there won 169,572 votes (21% of the vote).
When the electoral college cast its vote, all of the unpledged electors cast their votes for conservative Democrat Harry F. Byrd for President and Strom Thurmond for Vice President. They were joined by faithless Republican elector Henry D. Irwin, though Irwin cast his Vice Presidential vote for Barry Goldwater. Irwin had attempted to broker a coalition between the unpledged electors and other Republican electors, but to no avail: Kennedy won a majority of the electoral vote, though the odd situation with the mixed elector slate in Alabama makes it difficult to say that he won the popular vote.
The last slate of unpledged electors to date was filed in Alabama in the 1964 election. The slate was supported by Democratic Alabama Governor George C. Wallace, and the national Democratic nominee, Lyndon B. Johnson, did not appear on Alabama ballots. The electors won 30.6%, but the state was ultimately won by Republican Barry Goldwater.
Goldwater's victory in Alabama was a herald of a trend that would put an end to the practice of nominating unpledged electors. As a strategy, it had been largely ineffective, and southern conservatives, once loath to vote Republican, began doing so in increasing numbers. As such, the practice of nominating unpledged electors can be seen as a transitional phase between the Democrats' traditional hold on the south to the modern environment, where the area is a Republican stronghold.