Doubts are creeping in among opinion pollsters over how accurately they will be able to predict the outcome of a presidential election between Donald Trump Donald John TrumpObama calls on Senate not to fill Ginsburg's vacancy until after election Planned Parenthood: 'The fate of our rights' depends on Ginsburg replacement Progressive group to spend M in ad campaign on Supreme Court vacancy MORE and Hillary Clinton Hillary Diane Rodham ClintonWhat Senate Republicans have said about election-year Supreme Court vacancies Bipartisan praise pours in after Ginsburg's death Trump carries on with rally, unaware of Ginsburg's death MORE.

The two prospective nominees' high unfavorable ratings, and their fame, mean pollsters are facing one of their most difficult tasks in conducting polls that correctly model who will vote in November’s election.

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As many people are likely to come to the polls this fall to vote against the candidate they dislike as to vote for a candidate they support — something that makes polling difficult.

“We are looking at ... the most disliked candidates in a single election,” said Jennifer Dineen, a University of Connecticut polling expert and director of the school's joint poll with the Hartford Courant newspaper.

As a pollster, “you’re not determining who’s supporting a candidate; you are trying to determine how strongly someone opposes a candidate,” she added.

“People’s feelings on [Trump] are pretty clear, but we are not just trying to estimate support for Secretary Clinton, we are estimating opposition to him.”

People on the political left raised questions about polling accuracy after recent surveys showed the Republican businessman doing better against the Democratic former secretary of State than Beltway pundits expected.

A Fox News poll conducted in mid-May showed Trump leading Clinton by 3 percentage points.

It faced immediate pushback on social media from liberals who noted that the poll implied a turnout where self-declared Republicans outnumbered Democrats by a single percentage point, but Democrats held a 6-point advantage by that measure among actual voters in 2012.

Still, other polls that followed soon after have undercut the notion that the Fox survey was an outlier.

An ABC News/Washington Post poll put Trump up by 2 points. Other recent surveys have been more optimistic for Clinton — she was up by 3 points in an NBC News/Wall Street Journal poll and by 4 in a Quinnipiac University survey released Wednesday — but overall the race looks as if it could go either way.

The party identification of respondents in the ABC, NBC and Quinnipiac polls more closely tracked the actual 2012 turnout than did the Fox poll. Democrats had an edge of 8 points in the ABC survey and 6 points in the other two polls.

Experts note several caveats to all the current polls, however.

The fact that Trump has wrapped up the GOP nomination while Clinton is still trying to end an intraparty challenge from Bernie Sanders Bernie SandersKenosha will be a good bellwether in 2020 Biden's fiscal program: What is the likely market impact? McConnell accuses Democrats of sowing division by 'downplaying progress' on election security MORE could be providing the Republican with a bump, several experts said.

They also noted that polls this far out from Election Day have little predictive value, even if they are interesting as snapshots. This is because any number of future events before November could reshape the race and most people just aren't very focused on the contest yet.

“I would counsel that the American public comes in in tiers,” said Cliff Zukin, a polling expert at Rutgers University and a past president of the American Association for Public Opinion Research. Zukin compared the process to how sports leagues such as the NBA draw truly committed fans during the regular season, more people in the playoffs and the largest numbers of all to the championship series.

“It makes little sense to regard the public as a single whole. There is a certain audience for primaries, then historically the conventions have produced another tier of the electorate coming in. ... And then the last tier comes in, which traditionally is not until just before the election.”

Even the experts disagree over how antipathy toward both Trump and Clinton will affect who shows up at the polls in November.

Republican pollster David Winston leans toward low turnout, suggesting that the feelings of political partisans shouldn’t be confused with the view of the general public.

“Among the party folks, you have the dynamic where Trump’s best asset is Hillary Clinton and Clinton’s best asset is Donald Trump. If you are a Republican, Hillary Clinton is a motivator. If you are a Democrat, Donald Trump is a motivator,” Winston said. “But to those people who aren’t affiliated with a political party, that becomes a very different dynamic.”

Dineen, however, noted that Clinton and Trump are almost universally known and added, “People are most likely to turn out to prevent a candidate they are strongly opposed to from getting into office. ... They may engage more.”

There are other complications, too.

Karlyn Bowman, a polling expert with the conservative American Enterprise Institute, suggested that Trump’s divisiveness and unpopularity with many pundits could lead to respondents being reluctant to tell pollsters they actually support him.

“It’s really hard to gauge a closet vote,” Bowman said, referring to past contests such as Doug Wilder’s narrower-than-expected win in a 1989 Virginia gubernatorial election. On that occasion, polls appeared to underestimate opposition to Wilder, a Democrat who became the nation’s first elected African-American governor.

There is also the most basic problem of all. Trying to predict who will cast ballots in a future election inherently involves some element of guesswork.

“The problem of turnout is: How do you weigh something that does not yet exist?” Zukin said.

For all the uncertainty, pollsters can take heart from at least one development: They performed well, by and large, during the primary season, which is historically more challenging for pollsters because allegiances within a party flip much easier than in a general election.

There were some upsets that left pollsters red-faced, such as Sanders’s defeat of Clinton in Michigan — she had led by 21 points in the final RealClearPolitics polling average — but they were relatively few in number.

But the general election, typically decided by only a few points one way or another, could be a different matter.

A Trump-Clinton match-up, Bowman said, “could just be throwing out some of the old rules.”

This story was updated at 10:29 a.m., June 3.