Bernie Sanders wants to transform how the Democratic Party chooses its presidential nominee.

To do so, Sanders has made abolishing the closed primary — which prevents independents from voting — one of his top demands of Democratic officials, arguing that the party needs to "open the doors to working people, to senior citizens, to young people."

Expanding voter participation is indeed a noble goal. But if Sanders really wants the Democratic Party's presidential primaries to be more representative of the public, he should forget the closed primary and go after a much better target: the caucus.

On Thursday, the Lawyers' Committee for Civil Rights published a reportshowing that caucuses — which often require voters to spend hours at the polls — have been closely connected with very low voter turnout during this election.

The map and chart above make clear just how stark the divide is between caucus states and primary states. It's not just the raw mean average: Every caucus state but Idaho has seen turnout under 16 percent. Every primary state, including closed primaries, has seen turnout above 18 percent.

And it's not just that many experts think caucuses do more to restrict voter participation than closed primaries: There are also just way more of them. So far, 15 states have held caucuses so far. Only six have held closed primaries.

Of course, though it's hard to say that caucuses caused the lower state-by-state turnout, there is certainly a correlation.

The average turnout in states with primaries throughout 2016 has been 32.4 percent, compared to just 9.9 percent for caucus states, according to the Lawyers' Committee for Civil Rights.

"The caucus method of voting consistently produces lower voter turnout than the primary election method of voting," the committee notes in a press release. "The comparisons are striking."

Why are caucuses more restrictive than closed primaries?

Primaries work like general elections: Voters show up, enter a private voting booth, and cast a ballot for their preferred candidate. The whole process can take about 15 minutes, says Michael McDonald, who runs the United States Elections Project, which tracks voter turnout.

Closed primaries do throw up real obstacles to voter participation. Sometimes, as happened in New York, the deadlines to switch party affiliation can be so early that voters realize too late to participate.

But caucuses are "clearly" the most restrictive form of primaries, McDonald says. Not only are some of them also closed to independents, but they also require voters to devote a substantial portion of their day — sometimes as much as several hours — to participate.

"It's not an in-and-out process," McDonald says. "Primaries can be relatively small bites of your day; caucuses take time."

In the caucus rooms, attendees take turns arguing for their preferred candidate, and a voter's choice is often public to everyone who attends the caucuses. Caucus-goers have to show up at a specific time — often at night and during the work week. The process can take hours.

That means caucuses traditionally do away with absentee and other early voting options. "You have to dedicate a time to be physically in a location," McDonald says.

A second, and likely more serious problem, is that caucuses are often held when people can't escape work, childcare, or their other daily responsibilities.

"If you have a job or have kids or are in the military and have to be at your station — tough luck, you can't participate," says McDonald, who is also a political science professor at the University of Florida.

It's not as if we haven't known this for a long time. As far back as 1976, turnout across all of the presidential primaries was 1.9 percent for caucuses and 28.2 percent for primaries, according to a research paper by Harvard professor Thomas Patterson.

Sanders's case for abolishing closed primaries makes much more sense when applied to caucuses

There's an obvious and understandable reason Sanders has focused on reforming the closed primary instead of the caucus: Sanders has excelled in the caucus states, which reward fired-up activists, and suffered in the closed primaries, where his independent supporters can't participate.

Sanders loses about 5 percentage points in closed primaries compared to open primaries, according to an analysis by Emory political scientist Alan Abramowitz. Meanwhile, he's won big in caucus states like Minnesota, Washington state, and Kansas.

"Caucuses attract not just a smaller group of voters, but a group that is the most committed and ideological," write Brigham Young University political scientists Christopher Karpowitz and Jeremy C. Pope in the Washington Post.

Perhaps that's a worthwhile goal: One could defend having the party's most determined, committed voters having an outsize role in picking its nominee. "Some might say: 'that's good, you get the people involved in the party and really dedicated to the party,'" says Eric McGhee, a research fellow on electoral reform at the Public Policy Institute of California.

But McGhee says caucuses are ultimately incompatible with the goal of expand voter participation.

"Caucuses make it so hard to participate that nobody does," McGhee says. "Ultimately, if you're looking to get the support of the broader community of people you might as well involve them in the process of choosing your nominee."

This is where our modern primaries come from