In Texas, Hillary won the primary by 4% but Obama won the caucus by something like 12%. Why such a big difference in voter preference on the same day in the same state? The answer seems to be that caucuses attract a much smaller group of people; they attract a particular kind of person (hard-core activists) and discourage other types of voter; and caucuses are more open to shennanigans. In addition, at least in Texas, caucuses carry a disproportionate weight.
Overall, at this point, Obama has won eleven caucuses and Hillary has won one. In caucuses, Obama has won 65% of delegates and Clinton has won 35%. But in primaries, the two candidates have divided the delegates 50-50. (These results reflect, in part, that caucuses are more common in small-population states, and Obama has had more success in those spots, partly because those caucuses occurred during his period of momentum.)
Primaries are an election: people show up and mark a ballot. (However, there are several ways they can work.) Caucuses are a group thing: voters meet in a room, have some discussion, and then vote. Caucuses can be unpleasant to attend: they are often long, involve a lot of standing and can be chaotic. Here's a first-hand account of the Texas caucus last week. Caucuses appeal to younger, more strident people, and they discourage older people, people with busier schedules and people who shy away from conflict. Caucuses attract far fewer voters than primaries; in Texas, about 40,000 people voted in caucuses while nearly 3 million voted in the primary.
A difference that is frequently cited between primaries and caucuses is that anyone can vote in a primary, while only party members can vote in a caucus. This is untrue. In most primaries, only party members (or party members and independents) can participate. And some caucuses are open to everyone, as in Texas.
Caucuses are open to more irregularities than primaries. As one commenter to a blog post on the topic put it, a caucus: "1) Is designed for hard core activists, 2) Is easy to manipulate by those running it, 3) Is going to produce undemocratic results, 4) Is going to ensure that conspiracy theories abound when things are close, and 5) Is set up to make people feel empowered when they are really just suckers, the way that day traders are generally just suckers for the real Wall Street pros."
But caucuses are also more participatory. One reason that the Iowa caucus is so important is that the caucus system demands that the candidates make their positions on the issues clear. If I were a party activist in a caucus state, I could imagine feeling disenfrachised if the caucus system was replaced with a primary. On the other hand, if I lived in a state without a primary, I might feel disenfrachised because the only way I could vote would be to attend a protracted, potentially unpleasant public meeting.
Still, the results in Texas indicate that something is very wrong with the system. Texans were able to vote in both the primary and caucus. One of the problems is that the caucus, with less than 2% of the voter turnout of the primary, chose over one third of the delegates. These are the final numbers for Texas:
Primary (126 delegates)
Hillary: 1,459,814 votes (51%) - 65 delegates
Obama: 1,358,785 votes (47%) - 61 delegates
Caucus (67 delegates)
Hillary: 18,620 votes (44%) - 29 delegates*
Obama: 23,918 votes (56%) - 38 delegates*
Hillary: 94 delegates
Obama: 99 delegates
What the picture looks like if you combine the popular vote of the primary and caucus
Hillary: 1,478,434 (52%) - would be 100 delegates
Barack: 1,382,703 (48%) - would be 93 delegates
* The caucus delegates will be finalized at the state convention in June, but I estimated the numbers based on percentage of popular vote.
** Texas sends 228 delegates to the convention. This breakdown refers to the 193 that are pledged. There are also 35 superdelegates.