With many voters disappointed with their choices for president in the presumptive nominees of Hillary Clinton and Donald Trump, many are considering alternative candidates. Thus far, the greatest beneficiary of this disappointment has been Libertarian Party candidate Gary Johnson, who is consistently polling in double digits in three-way contests with Clinton and Trump.

Johnson's growing popularity is alarming many conservatives, who fear that Johnson could play the role of "spoiler" by siphoning votes from Trump in November and thus enabling the election of Clinton.

Johnson has rejected the label. "I don't think I'm going to be a spoiler in this," he told Meet The Press on June 5, adding, "I think it's a draw from both sides."

Some polling confirms this expectation. A Fox News poll from May, for instance, showed that Trump led Clinton in a two-way contest, 45 percent to 42 percent; yet when Johnson was added as an option, he polled 6 percent, drawing 3 percentage points from both candidates.

Recent history also shows that libertarian candidates attract both liberal and conservative voters.

Prior to the 2013 gubernatorial election in Virginia, Libertarian Party candidate Robert Sarvis received criticism from conservatives for threatening to "steal" votes from Republican candidate Ken Cuccinelli to the benefit of Democratic candidate Terry McAuliffe. Sarvis ended up receiving 6.5 percent of the vote, and McAuliffe won the election by a narrow margin of 55,000 votes – far fewer than the 146,000 that went to Sarvis. Some attributed Cuccinelli's loss to Sarvis' candidacy and even floated the idea that he was a Democratic Party "plant."

But post-election analysis contradicted the claim that Sarvis spoiled the election for Cuccinelli. Exit polls showed that 7 percent of liberals supported Sarvis compared to 3 percent of conservatives and 10 percent of moderates – evidence that libertarians do appeal to "both sides." Moreover, Sarvis supporters tended to profess McAuliffe, not Cuccinelli, as their second choice.

Nevertheless, the belief that libertarians hurt Republicans persists on the right. One prominent conservative has argued that "The Libertarian Party could play the spoiler role in 2016 for Donald Trump, just as Ralph Nader did in 2000."

Ralph Nader, of course, is often cited as the textbook "spoiler" for his alleged role in stealing votes from Al Gore in Florida in 2000, a state which Bush won by a razor-thin margin of 537 votes – delivering him the presidency.

Yet the then-Green Party candidate insists that his 97,000 Florida votes are not what cost Gore the election. He faults Gore for losing in his home state of Tennessee, which he ought to have won given his "home field advantage," and in Florida, Nader observes that Gore failed to maintain the support of a majority of the state's registered Democrats, half of which did not turn out to vote and at least 200,000 of which voted for Bush.

Nader also points to other idiosyncrasies of the 2000 election, including the mistaken purging of nearly 100,000 voters from the voting rolls, the use of confusing butterfly ballots in Palm Beach County, incomplete vote recounts, and partisan rulings culminating the 5-4 decision from the U.S. Supreme Court ordering the cessation of the recount.

Nevertheless, Nader did draw votes away from Gore in Florida. Exit polls have shown that had Nader not appeared on the ballot, approximately twice as many Nader supporters would have voted for Gore than Bush.

Yet Nader disputes the notion that third party candidates are, or ever can be, spoilers. "All candidates on the ballot try to get votes from one another. Either they are all spoilers, or none of them is," Nader wrote in an op-ed in the Los Angeles Times on June 10. "Arrogantly applying that word only to minor party candidates is to treat them as second-class citizens and set them up as scapegoats in close elections."

Nader argues that constantly voting "strategically" for the "lesser of two evils" perpetuates the unpopular two-party system, with profound consequences: broadly disliked candidates with no incentive to improve or compete, a frustrated and apathetic electorate, and the empowerment of politicians who continue to thwart third party and independent candidacies and stifle voter choice.

Instead, Nader counsels the electorate to vote 'for' rather than said on C-SPAN on May 4. "If they vote for the least-worst, they'll never have any leverage."

Yet the leaders of the Democratic and Republican parties are using fear of the other party's nominee to encourage such strategic voting. RNC Chairman Reince Priebus has called the #NeverTrump movement's effort to recruit an independent candidate a “suicide mission for our country" given the prospect of a Clinton presidency, and the DNC, eager to "unify and take on" Trump, is aggressively trying to appease Sanders and his supporters post-primary season to stymie the appeal of alternatives such as Green Party candidate Jill Stein.

The debate over whether third party and independent candidates are spoilers depends on one's beliefs about to whom a citizen's vote belongs: those outside the political duopoly insist that a vote should be cast for the candidate whom the voter wants to see elected, whereas Republicans and Democrats presume that votes "belong" to them, and that votes for an alternative candidate amount to "wasted" votes.

With unfavorability ratings at an all-time high for Clinton and Trump, it is up to voters to decide which approach to voting they will adopt this November: will they vote their conscience for their preferred candidates, or will they reject them as the spoilers that the major parties advertise them to be?