What the Democratic primary polls might be missing

The 2020 Democratic presidential primary appears to be a three-candidate race, with former Vice President Joe Biden, Sen. Elizabeth Warren, and Sen. Bernie Sanders being the only candidates polling in the double digits. Averages suggest Warren and Biden are more or less tied for the lead nationally, with Sanders trailing but well ahead of the rest of the pack.

There are many reasons to think polling might not fully capture the current state of the race, however. Due to the dramatically different makeup of their supporters, each candidate has a separate and compelling case for why they could end up doing substantially better than what the polling says.

The Sanders case: His base is hard to poll

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Both national and early state polling point to the fact that Sanders' coalition is significantly younger, less educated, lower income, and less likely to be paying attention to political news. For example, in a recent Quinnipiac national poll, Sanders is polling at 39 percent with voters 18-34 compared with Biden who polls at just 3 percent with this group. And Sanders is also overwhelmingly winning among those paying little or no attention to the campaign.

This makes for a coalition which is overwhelmingly made up of exactly the type of people who are the hardest to poll. It is also hard to predict if these voters will turn out. A major thrust of the Sanders campaign is about "building a movement," with one of their major goals being to bring new and marginal voters into the process to get them to turn out for Sanders. If the campaign can succeed at this, which is a big if, it is conceivable their election results could exceed their polling. Last cycle, Sanders did significantly outperform polling in certain primary states like Michigan and his campaign excelled at getting supporters to disproportionately turnout for caucuses. So there is reason to believe he can.

The Biden case: His supporters are the most reliable voters

The case for Biden outperforming the polling is basically the exact inverse of Sanders' case. In polling, Biden dominates with older voters. For example, a recent CNN poll shows Biden with a 14-point lead among seniors in Nevada, and an Economist/YouGov poll shows him with a 12-point lead nationally among those over 65. Seniors are by far the most reliable voting age group, and there is little doubt these seniors, who overwhelmingly support Biden at the moment, will turn out. If for any number of reasons turnout in the primaries ends up lower than expected, Biden would likely outperform the polls due to who his voters are.

The Warren case: She's everyone's second choice

At the moment, Warren benefits from the fact that primary voters really like her, even if she isn't their first choice. The Economist/YouGov poll found Warren has the highest number of Democratic primary voters considering her at 58 percent, 10 points higher than Biden in second place.

Two recent Iowa polls and a Nevada poll all found that Warren was the most popular second choice candidate. The national Morning Consult poll also found Warren is the clear second choice for both Kamala Harris supporters and Pete Buttigieg supporters. This could be decisive because in Iowa and Nevada, if a candidate doesn't have enough support at individual caucus locations, those supporters can go align with their second choices. Based on their current polling, Harris and Buttigieg might have a decent but insufficient level of support at many caucus locations.

It is easy to see how Warren could end up with a large last-minute surge that would mostly be missed by the polling. She is well positioned to pick up the supporters of trailing candidates who may drop out in the next few months or those who may look to jump ship right before the actual primary vote or during the caucus process.

Which one of these different dynamics will prove true? Perhaps none, or perhaps all three. They also may only have a small impact, might cancel each other out, or one could prove to be decisive. The thing to remember is that it wouldn't be surprising if one or even two of the candidates ended up doing surprisingly better than expected once the voting actually starts.

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