Nearly half, 47 percent, of registered American voters would consider a third-party candidate if Hillary Clinton and Donald Trump were the major party nominees, according to a new NBC/WSJ poll.

Support for third parties is steadily growing. Similar polls conducted during past presidential elections found that in 2012, 40 percent of voters would support a third-party candidate, and in 2008, 38 percent of voters would.

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The 2016 presidential campaign has inspired intense backlash against the establishment. Bernie Sanders has attacked it from the left, for kowtowing to corporate power and repressing grassroots activist movements. Donald Trump has attacked it from the right, pledging to restrict immigration and fueling racist xenophobia.

Left with the prospect of a Hillary Clinton or Trump presidency, some Americans have called for third-party presidential runs.

The Republican Party establishment, much of which mistrusts Trump for his volatility and frequent flip-flopping on issues like trade, health care and foreign policy, has considered a third-party bid.

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Former GOP presidential candidate Mitt Romney considered running as an independent, although he later backed off. Still, popular right-wing websites have called for a right-wing third-party alternative to Trump.

On the other side, at least one-quarter of Sanders voters say they refuse to back Clinton, an extremely hawkish and extremely wealthy multimillionaire who is Wall Street's favored candidate. Supporters of the "Bernie or Bust" movement have asked the Vermont senator and self-declared democratic socialist to run third party.

Sanders himself has rejected calls for a third-party run. Yet, simultaneously, his campaign has constantly stressed that poll after poll show he is most well-poised to defeat Trump.

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The same NBC/WSJ poll that found nearly half of voters would support a third-party presidential candidate also found that Clinton and Trump are neck-and-neck nationally.

Clinton leads the far-right presumptive GOP nominee by just 3 points, with 46 percent to 43 percent, according to the poll, which was conducted from May 15 to 19, with a sample size of 1,000 registered voters.

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It also found that Sanders leads Trump by a strong 15 points, with 54 percent to Trump's 39 percent.

Other polls have found that Trump is even beating Clinton on the national stage.

These signs have reignited calls for a third-party alternative to both Clinton and Trump.

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Green Party presidential candidate Jill Stein, who also ran in the 2012 campaign, condemned the fact that third-party candidates are increasingly popular, but are still largely excluded from polls and other and shut out from political institutions, calling it "beyond unacceptable."

Stein was recently interviewed by popular video news site AJ+. "People are hungering for more choices, but the American political system excels at suppressing the voices of opposition," she said in the segment.

"In fact we are probably the only major developed country that is limited to two parties," Stein added.

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"People are told over and over: don't vote your values, vote your fears. But what we got was everything was everything we were afraid of," the Green Party presidential candidate explained.

"All the reasons you were told that you had to vote for the lesser evil, because you didn't want the massive expanding wars, you didn't want the meltdown of the climate, you didn't want the massive Wall Street bailouts, you didn't want the offshoring of our jobs."

"These are all things that were brought to us — signed, sealed and delivered — by a Democratic White House with two Democratic houses of Congress," Stein continued.

"We have a few things to learn about democracy in our own country, that we need to allow opposition voices and opposition political parties. That's how we get to real democracy."