Since the 1856 election, all U.S. presidents have been Republicans or Democrats, but that hasn't stopped third-party candidates from trying.
Their chances of winning the keys to the Oval Office may be slim, but the also-rans could be more popular than ever in 2016. Polls show far more voters hold strongly unfavourable views of the two presumptive nominees compared with other Democrat and Republican candidates in recent decades. That has spawned a so-called "Bernie or Bust" movement by Bernie Sanders supporters who dislike Hillary Clinton, and the "Never Trump" movement among disaffected Republicans looking for an alternative to Donald Trump.
Even if they don't win, third-party nominees sometimes score enough of the popular vote to potentially change the outcome. For example, George Bush squeaked by in Florida in 2000, beating Al Gore by just 537 votes to ultimately win the presidency.
The fact Green Party candidate Ralph Nader received more than 97,000 votes in Florida that year caused some to question whether he had split the vote and robbed Gore of a crucial win that would have landed him in the White House.
In the run-up to the presidential election in November this year, three third-party candidates are getting the most attention. Here's a look.
Gary Johnson, The Libertarian Party
Johnson was a two-term Republican Governor of New Mexico and his running mate, Bill Weld, was Governor of Massachusetts for six years.
Under Johnson's leadership, the Libertarians were the most popular third-party nationwide in 2012, earning 1,275,971 votes. That was just shy of one per cent of the total.
The party is in favour of smaller government, lower taxes and free markets -- three things that Republicans often say they support but don't always put into practice when elected.
The fact that Trump has broken with his fellow Republicans on free trade could make the Libertarians especially attractive to fiscal conservatives and other Never Trumpers.
Johnson could also draw votes from people offended by Trump's comments about Mexico sending criminals and rapists across the U.S. border. The former governor has called the comments "just racist" and pointed to statistics showing immigrants commit fewer crimes than native-born Americans.
Jill Stein, Green Party
A Harvard graduate and physician, Stein was the Green Party candidate in 2012 when it won 469,627 votes for president -- 0.36 per cent of the total -- making her the fourth-place candidate.
That was the best result for the environment-focused party since Nader ran as a Green in 2000, getting 2,882,955 votes nationwide or 2.74 per cent of the total.
Pundits have suggested Stein's positions are close enough to Bernie Sanders that she could get a lot of the so-called "Bernie or Bust" voters -- she is a critic of capitalism and favours single-payer health care, for example.
Only 50 per cent of Sanders supporters recently polled by YouGov.com said they would vote for Hillary Clinton if she is the party's nominee.
Darrell Castle, The Constitution Party
A Tennessee native, Vietnam veteran and lawyer, Castle has won the nomination of the party that got the fifth largest number of votes in 2012 -- 122,389 or 0.09 per cent.
The party has "seven principles" based on the themes of life (it is anti-abortion), liberty (it opposes gun control), family (it calls LGBT people "sexually deviant"), property rights, state rights and "American sovereignty."
In addition to the U.S. Constitution, the party also bases its beliefs on the Bible. That could help it win over conservative Christian voters who would normally choose a Republican but are turned off by Trump's inconsistency on such "family values" issues as LGBT rights and abortion.
Although Trump has said he would appoint judges to overturn the Supreme Court ruling that legalized same-sex marriage, he has said his "friends" Elton John and David Furnish have a marriage that "is going to work," and that transgender people should be able to use whatever bathroom they feel is appropriate.
Trump told NBC in March that he is "pro-life" and supports "punishment" for women who have abortions, but he told NBC in 1999 that he hated abortion but was "very pro-choice … pro-choice in every respect."
