Like most political science concepts, the term “open primary” is a bit confusing and isn’t always used consistently. Technically, an open primary is one in which you don’t have to be affiliated with a political party to vote in that party’s primary — but, because states have all kinds of different rules about the technicalities of this process, political scientists (or whoever it is that comes up with these terms) have differentiated between open primaries, closed primaries, semi-open primaries, semi-closed primaries, and nonpartisan blanket primaries, sometimes referred to as “jungle primaries” (I’m probably also leaving a few subcategories out). Adding to the confusion is that some states allow same-day voter registration, which means that even if a primary is closed (or semi-closed), voters can switch parties on election day and vote in whatever primary they want.
Essentially, what it comes down to is that some states are friendlier to independents and cross-party voters, meaning that in these states, independents and Republicans can cast a ballot for a Democrat. This is especially significant for 2020, because the Republican primary won’t really matter — not only will Donald Trump as a sitting president obviously comfortably win, but some states (specifically Alaska, Nevada, South Carolina, and Kansas) aren’t even bothering to hold Republican primary elections at all. What this means is that many Republican and independent voters who voted for Trump in 2016 could, in the open (and semi-open) primary and same-day registration states, decide to vote for a Democratic candidate — if they find that candidate appealing.
There is, of course, one candidate running in the Democratic Party whose populist appeal extends to independents, traditional non-voters, and even Republicans, as evidenced by his warm reception by the audiences of both the Fox News town hall he did back in April and Joe Rogan’s podcast, which he appeared on in August. In 2016, Bernie Sanders surprised pundits by doing far better than expected in the Democratic primary, especially given that Clinton had done her best to wrap up the process before voting even began by solidifying endorsements from almost every party leader across the country. In particular, Sanders did well with young people, the working class, rural voters, and independents (in Iowa, he won 69% of independents to Clinton’s 39%) — and while he didn’t ultimately defeat Clinton, he brought, for the primaries at least, new voters into the Democratic party and made the primary election much closer than expected.
In the 2020 primary, Bernie will once again, in open (and semi-open) primaries and in same-day registration states, draw support from independents and Republicans — except this time, given the lack of a Republican primary, it’s likely he’ll draw even more than he did before, since they have no faux-populist Republican alternative actively courting them. Instead, they can be swayed by Bernie’s genuine populist message.
Iowa is a state with a semi-closed caucus (although some people sometimes call it semi-open, which is confusing) — but importantly, they have same-day voter registration, which means Republicans and independents can register as Democrats to vote for Bernie in the caucus. These voters are, of course, not considered “likely voters” by pollsters — they’re not Democrats, and so some pollsters assume they won’t vote in the Democratic primary. But many of them did vote in the Democratic primary in 2016, and overwhelmingly for Bernie Sanders — that’s how he was able to get 49.59% of the vote in Iowa. The reason Sanders’s numbers have seemed so much lower in recent Iowa polls (something Nate Silver of FiveThirtyEight loves to gleefully point out on Twitter), in particular in the recent Des Moines Register poll, which had him at 11%, is that pollsters are only asking “likely Democratic caucus goers,” who the pollsters assume are overwhelmingly registered Democrats. But because the Iowa caucuses have same-day registration, many independents who don’t appear in these polls will likely vote for Bernie, as happened in 2016.
If we look back at 2016, Bernie won most states with open primaries or same-day voter registration, or else came within less than a percentage point, as happened in Iowa and Missouri. The exceptions were southern states, where Bernie struggled with black voters, Illinois, which was one of Clinton’s home states (although Bernie did come very close, with 48.61% of the vote), and finally California, which of course didn’t vote until after Clinton had already (thanks to superdelegates) obtained a delegate majority, a factor which suppressed turnout (again, though, given the fact that the primary was already over, Sanders came remarkably close even here, with 46.04%).
Ultimately, what all this means is that Bernie supporters should take heart. Don’t be discouraged by a few early state polls where he ends up in third. His appeal is primarily to non-traditional voters, to independents disillusioned with the two-party system, to young people disillusioned with politics in general, and to working-class Republicans who were seduced by Trump’s pseudo-populist message and who, because they’re suffering in this horribly rigged economy, are looking for a candidate who will genuinely speak to their needs. The open (and semi-open) primary system and the existence of same-day registration laws mean that Bernie will end up with a much higher percentage of the final vote than these polls show. Of course, not every state has such liberal voting laws — ironically, New York, one of the bluest of blue states, has an incredibly regressive system: the deadline to register to vote in the April 2020 primary is, inexplicably, October 11. But hopefully by then Bernie will have won Iowa and New Hampshire because of their semi-open primaries, as well many of the Super Tuesday states, and he’ll have the momentum to carry New York too.
As for the pundits who are writing off Bernie’s chances — please do a little research and recognize the limitations of these polls you take as oracles.