Both of the major party presidential candidates are hideously unpopular, and many Americans are thinking about voting for a third-party candidate in November. The idea is that by voting for somebody other than Donald Trump or Hillary Clinton, disaffected voters can send a message to the major parties and compel them to make changes to their platforms. This is a fallacy: Voting third-party doesn't send a message. Historically, it never has sent a message, and it will not send a message in the 2016 election.
In most presidential elections, the third-party has a negligible effect on the outcome. A couple of times, however, third-party candidates have drawn enough support to either win several states or play a "spoiler" role and swing the election to one of the major party candidates. In the last century, this has happened in three elections: 1948, 1968, and 2000. In none of those cases did either of the two major parties, or the incoming president, make changes to their policies based on the third-party vote.
In 1948, President Harry Truman was running for reelection against Republican Thomas Dewey. They were joined by Strom Thurmond, who ran as a candidate for the States' Rights Democratic Party. Thurmond opposed school desegregation, supported Jim Crow laws and would go on to filibuster the Civil Rights Act in the 1950s. His candidacy was, in essence, centered on opposition to civil rights.
The expectation was that Thurmond's candidacy would split the Democratic vote — at that point in time, Democrats were largely opposed to racial integration — and hand the election to Dewey. At the very least, a strong showing by Thurmond might convince the next president to give greater consideration to his cause.
As it turns out, Thurmond won four Southern states, plus one electoral vote in Tennessee. But Truman won the election, ignored Thurmond's policy prescriptions completely and pursued a pro-civil rights agenda. The next president, Republican Dwight D. Eisenhower, also adopted none of Thurmond's recommendations and went on to sign the 1957 Civil Rights Act. If Thurmond and his historically-successful third-party campaign did send a message, nobody of influence received it.
Twenty years later, Alabama Gov. George Wallace launched a third-party candidacy against Richard Nixon and Hubert Humphrey. Like Thurmond, Wallace was a staunch segregationist and opponent of civil rights. He called for the United States to withdraw from Vietnam and, in general, opposed federal encroachment on states' rights. And his message resonated, to a degree: Wallace won five entire states in the general election, the last time a third-party candidate has won any state.
Yet Wallace's campaign had little to no impact on the direction of the country's policies. Nixon won the election and went on to desegregate the schools, expand the scope of the federal government enormously, and prolong the war in Vietnam. No message sent.
Wallace and Thurmond both won states in their own right, but that's not the only way a third-party candidate can have an impact.
In the 2000 election, Green Party candidate Ralph Nader didn't win any states. However, he won enough votes in two states (Florida and New Hampshire) to, arguably, swing the election to George W. Bush. The question of whether Nader is to "blame" for Bush's victory is still contentiously debated. What's indisputable, though, is that Nader received 97,488 votes in Florida, and Al Gore lost (officially) by 537 votes. If just half of one percent of the left-leaning Nader voters had punched their ballots for Gore instead, Bush would have never become president.
But Bush did become president, and did more or less the exact opposite of everything Nader proposed during the campaign. Nader advocated for environmental protections, stronger regulatory agencies, and gay marriage legalization; Bush decimated environmental laws, deregulated everything, and attempted to ban gay marriage with a Constitutional amendment.
For voters who legitimately don't have a preference between the two major party candidates, voting third-party makes sense and feels good. But make no mistake: Third-party votes have never effectively sent a message in a presidential election, and there's no reason to think they will in 2016, either.