The self-described democratic socialist, Bernie Sanders, the independent senator from Vermont is not a member of the Democratic Party although he running is for that that party's nomination. It is probably not a coincidence that he does best in the nominating contests that are the least Democratic and the least democratic.
Caucuses are inherently undemocratic
Primaries, especially those that allow voters to vote by mail or vote early, are the most democratic of the nominating contest held by the states in the run up to the general election.
Caucuses exclude most voters. For example, just 15.7 percent of Iowa's voting-eligible population participated in the caucuses in 2016. A similar percent of participation is being reported for the 2020 Iowa caucuses. Iowa's caucus turnout is less than a third of other states' contested presidential primaries. In 2016, 52 percent of New Hampshire voters turned out for that state's primary.
Access to the caucuses is a problem for more than those with disabilities. There are obstacles to participating in caucuses for the elderly, for those who don't speak English, for parents of young children who can't afford or find a babysitter, and for shift workers who can't take time off work to caucus for hours. Hard-working people who come home from work exhausted may not have the energy to caucus.
When we vote in the general election we are not expected to justify our vote. We might vote for somebody because of a single issue or multiple issues. We might vote for a candidate simply because we like their looks. We might decide on a candidate because of a "second sense" about their trustworthiness. The bottom line is that we can vote for a candidate based on the most illogical of reasons. Whatever our reason, we don't necessarily want to argue the point with our neighbors.
There is a lot to be said for the secret ballot.
Not everybody has the inclination to spend an evening browbeating or cajoling their neighbors into supporting a particular candidate. Not everybody wants to be browbeaten or cajoled. Some of us prefer to keep our political views private.
Open nominating contests incentivise political mischief
When a party is selecting a candidate to be their standard-bearer, the opposing party has an incentive to meddle in that process. Open nominating contests permit voters to cast a vote across party lines. In the case of Iowa, where for Republicans it was foregone conclusion that Trump would win all of Iowa's 2020 delegates, they could engage in political mischief by participating in the Democratic caucuses and advocate for the Democratic candidate who they judged to be the weakest in the general election.
Here are the various types of nominating contests in worst to best order.
Open Primaries and caucuses
States that do not ask voters to choose parties on the voter registration form are "open primary or caucus" states. In open primaries and caucuses, voters may choose privately in which primary or caucus to vote. In other words, voters may choose which party's ballot to vote, but this decision is private and does not register the voter with that party. This permits a voter to cast a vote across party lines for the primary or caucus election. Critics argue that the open primary or caucus dilutes the parties' ability to nominate. Supporters say this system gives voters maximal flexibility—allowing them to cross party lines—and maintains their privacy. In the 2016 open primaries Sanders won five of 17, a winning percent of 29%. He won all four open caucuses.
Semi-open primaries and caucuses
This system permits voters to cross party lines, but they must either publicly declare their ballot choice or their ballot selection may be regarded as a form of registration with the corresponding party. Iowa asks voters to choose a party on the state voter registration form, yet it allows a primary or caucus voter to publicly change party affiliation for purposes of voting on primary or caucus Election Day. Some state parties keep track of who votes in their primaries and caucuses as a means to identify their backers. Sanders lost the one semi-open primary. He split on the two semi-open caucuses.
Semi-closed primaries and caucuses
In this system, state law permits political parties to choose whether to allow unaffiliated voters or voters not registered with the party to participate in their nominating contests before each election cycle. In this type of system, parties may let in unaffiliated voters, while still excluding members of opposing parties. This system gives the parties more flexibility from year-to-year about which voters to include. At the same time, it can create uncertainty about whether or not certain voters can participate in party primaries and caucuses in a given year. In the semi-closed primaries Sanders won four out of nine, or 44% of them. He won the one semi-closed caucus.
Closed primaries and caucuses
A voter seeking to vote in a closed primary or caucus must first be a registered party member. This system deters "cross-over" voting by members of other parties. Independent or unaffiliated voters, by definition, are excluded from participating in the party nomination contests. This system generally contributes to a strong party organization. Sanders lost 86% of the closed primaries, taking two out of the 14 primaries. He won six of the 11 closed caucuses, or 55% of them.