A voter holds a sample ballot with her grocery list scribbled on the front page after voting for the midterm elections at Los Angeles County Lifeguard headquarters on November 2, 2010.; Credit: Kevork Djansezian/Getty Images

The deadline to register to vote in the California primary is just one week away.

Voters hoping to weigh in on the contest between Bernie Sanders and Hillary Clinton might want to double-check their party affiliation. That’s because California has what’s called a “modified closed system.”

For more on what this means and how the system came to be, Take Two spoke to Thad Kousser, professor of political science at UC San Diego. 

What is a ‘modified closed system?’

This is one of a plethora of ways of running a primary election that California’s had. We’ve had four different changes in the rules of our primary system in the last two decades. The semi-closed works like this: any party that wants to open up its primary to people who don’t have a party — no party preference voters, which are about a quarter of the electorate — any party can say yes to them. The Democrats have said yes, the Republicans have said no, but those voters need to know that they need to ask for that ballot … and if you want to vote by mail that way you need to ask for that in advance, so you don’t get a ballot with no president [on it].

How did this all come to be?

It came from California’s Donald Trump moment. In 1992, a Senate seat opened up. On the Republican side, you had this Stanford professor: a moderate congressman named Tom Campbell, who ran against a talk radio host, this guy named Bruce Herschensohn, so he narrowly beats Tom Campbell, but he ends up losing to Barbara Boxer.

A lot of moderate Republicans who backed Campbell said ‘we gotta have a different system that’s more inclusive, so we don’t have a polarized figure winning. Let’s go to a blanket primary so anyone can vote for anyone.’ So the state passed that as a ballot initiative in 1996, it was instituted in 1998, two years later Supreme Court throws it out and says ‘parties need to have the right to associate themselves with whoever they want to be, not any single voter who walks into the polling place.’

Press the blue play button to hear more. (Responses have been edited for length.)