DES MOINES — Voters in large numbers do not approve of the presidential candidates presented to them by the Democratic and Republican parties.

That has given third-party candidates an opportunity to make more of an impact on this election than any since the 1990s.

Republican Donald Trump and Democrat Hillary Clinton are viewed less favorably by voters than any major-party presidential candidates since pollsters first asked the question in the 1960s. In a recent NBC News poll, Trump was viewed favorably by 36 percent of voters and unfavorably by 62 percent, and Clinton was viewed favorably by 38 percent and unfavorably by 60 percent.

That appears to have created an opening for candidates from outside the two major parties, including Libertarian Gary Johnson, Jill Stein of the Green Party and independent Evan McMullin.

“It’s a race like no other, in a sense that these are the two most disliked candidates that have run for president in recent memory, and some people are searching for some out, for some kind of third option,” said Kedron Bardwell, chair of the political science department at Simpson College in Indianola.

Johnson thus far has had the most polling success. He has reached double digits in many polls and has the best chance of the third-party candidates to qualify for the presidential debates.

To participate in the debates, Johnson must average 15 percent in polls approved by the debate commission. Johnson is averaging 9 percent in Real Clear Politics’ polling average.

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Third-party history

Third-party candidates have faced long odds in U.S. presidential elections ever since the Democratic and Republican parties took command in the late 1800s. No state has gone to a third-party candidate since 1968, when George Wallace won five states in the South, and no third-party candidate has cracked the top two in an election since Theodore Roosevelt, as a Progressive in 1912, finished a distant second to Democrat Woodrow Wilson but won six states and beat incumbent Republican President William Taft.

Third-party candidates most recently had tangible effects in the 1992, 1996 and 2000 elections.

In 1992, Ross Perot qualified for the debates as an independent and earned 17.4 percent of the vote in Iowa. He tried again in 1996 with the Reform Party that he created and got 8.5 percent of the Iowa vote.

In 2000, Ralph Nader of the Green Party got 2 percent of the vote. No third-party candidate has earned as much as 1 percent of the vote since.

Johnson and Stein, the two most prominent third-party candidates in the polls this year, were on the ballot in 2012 as well. Johnson secured just 0.8 percent of the vote in Iowa and Stein 0.2 percent.

‘Very low bar’

“There’s a very low bar for how well these candidates have done in the last few elections,” Bardwell said. “I don’t think a lot of people have that distinct memory of Perot, so they’re used to these elections where a certain number of people say they’re going to vote for third party, and then a week before the race comes, all of a sudden they say, ‘Oh, gosh, but that would waste my vote.’ So they change.”

It appears unlikely a third-party candidate this year will succeed in winning a state. If he continues to move up in the polls, Johnson could reach the level of support Perot reached in the 1990s.

But Johnson or Stein could affect the election by siphoning support from Trump or Clinton, tilting the scales of the presidential race.

Candidates unknown

Polling experts have said it is too early to tell whether Trump or Clinton are losing more voters to third-party candidates. What’s more, those experts say it’s not clear voters who say now that they plan to vote for a third-party candidate will do so when they cast their ballots.

“I haven’t seen (the polling impact of third-party candidates) shift a whole lot, and when I have seen it shift, I’ve seen it shift both ways,” said Chris Larimer, a political-science professor at the University of Northern Iowa. “I think it’s still a little bit of an unknown.”

The third-party candidates also remain largely unknown. On average, more than half of voters surveyed do not know enough about Johnson or Stein to form an opinion, according to the Huffington Post’s poll tracker.

Meet the candidates

GARY JOHNSON

Party: Libertarian

Age: 63

Running mate: Bill Weld, a former governor of Massachusetts

Number of states on the ballot: 50

Political experience: Johnson was twice elected governor of New Mexico as a Republican in 1994 and 1998. He switched his party affiliation in 2011 when he decided to run for president.

Johnson made a campaign visit to Iowa over Labor Day weekend. Speaking to a crowd of roughly 400, Johnson advocated for racial and marriage equality, a streamlined tax system, gun rights, term limits, choices in health care and education, reduced military spending, entitlement reform, legalized marijuana and expanded immigration.

JILL STEIN

Party: Green

Age: 66

Running mate: Ajamu Baraka, a human rights defender and grass-roots organizer

Number of states on the ballot: 44, including Iowa

Political experience: Stein ran unsuccessfully for several offices in Massachusetts for the Green-Rainbow Party, including governor (she lost to Mitt Romney in 2002), state representative and secretary of state.

Stein’s major play has been to supporters of former Democratic candidate Bernie Sanders who have not transferred their support to Clinton. Stein supports single-payer “Medicare for all” health care, tuition-free college, a $15-per-hour minimum wage, a 100 percent renewable energy grid by 2030 and enhanced environmental and anti-climate change efforts.

EVAN MCMULLIN

(No photo available)

Party: Independent

Age: 40

Running mate: Not yet named

Number of states on the ballot: Nine, including Iowa

Political experience: Served as senior adviser to the U.S. House Committee on Foreign Affairs and later became the chief policy director of the House Republican Conference.

A former CIA counterterrorism and intelligence officer, McMullin is a conservative independent candidate dismayed by Trump’s candidacy. McMullin’s own candidacy has not taken off; he has not registered in the polls in the same way as Johnson or Stein.

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Third-party history

Top third-party vote totals in recent presidential elections in Iowa:

Candidate (party) votes share of vote

2012:

Gary Johnson (Libertarian) 12,926 0.8 percent

Jill Stein (Green) 3,769 0.2 percent

2008:

Ralph Nader (Peace & Freedom) 8,014 0.5 percent

Bob Barr (Libertarian) 4,590 0.3 percent

Chuck Baldwin (Constitution) 4,445 0.3 percent

2004:

Ralph Nader (petition) 5,973 0.4 percent

Michael Badnarik (Libertarian) 2,992 0.2 percent

2000:

Ralph Nader (Green) 29,374 2 percent

Pat Buchanan (Reform) 5,731 0.4 percent

1996:

Ross Perot (Reform) 105,159 8.5 percent

Ralph Nader (Green) 6,550 0.5 percent

1992:

Ross Perot (petition) 235,468 17.4 percent