Christopher F. Karpowitz and Jeremy C. Pope are associate professors of political science and co-directors of the Center for the Study of Elections and Democracy at Brigham Young University. The opinions expressed in this commentary are theirs.

(CNN) With only a few days until the Iowa caucuses, the political world is buzzing with anticipation for the presidential nomination process to go beyond polls and speeches to actual voting and caucusing.

Paying attention to the difference between these two types of processes is important because caucuses attract a very different set of citizens -- more ideological, more consistent and more partisan participants. We think this difference will matter.

So far, people have focused on Donald Trump's unexpectedly strong showing in the polls and on tensions between "outsider" candidates (like Trump) and more traditional "establishment" candidates. On the Democratic side, the question is how the expected front-runner Hillary Clinton will fare against an insurgent campaign from self-declared democratic socialist Bernie Sanders. Now, the critical question for all of these campaigns is how to get their supporters to show up.

As illustrated in the most recent CNN poll, different assumptions about the set of participants make a big difference. For instance, in the Iowa caucuses, Trump's lead disappears when only considering poll respondents who have previously caucused.

Our recent research indicates that it's important to pay attention to such differences because the processes and institutions through which citizens express their opinions can make a major difference for who wins.

Iowans make their choices in caucuses -- meetings where people show up, talk about the candidates in a group setting and then make a choice. In Iowa, Republicans will cast ballots at the meeting, while Democrats will use a more public head count where others can see each participant's favored candidate. In contrast, New Hampshire voters will take part in a primary -- a more traditional election where people go to the polls and cast a private ballot in the voting booth.

Many more people show up to vote in a traditional primary than participate in a caucus, but the size of the electorate is not the only difference between these two institutions. Our research shows that those who attend caucuses are not only a smaller group; they are also much more ideologically extreme than voters who participate in primaries.

In other words, primary voters are more moderate, and caucus participants tend to be much more consistently liberal or conservative. In fact, even after accounting for many other factors, we found that caucus attendees looked a lot more like more ideological members of Congress than they did average Republicans or Democrats.

This difference was on clear display in the 2008 contest between Hillary Clinton and Barack Obama. It was clear that Obama's supporters were generally more liberal than were Clinton's but it was also clear that for both candidates, supporters who attended caucuses were more liberal than were the candidate's supporters who voted in primaries.

In 2008, Barack Obama's margin of victory over Hillary Clinton for the nomination of the Democratic Party was largely a result of his stronger showing at the caucuses (places like Iowa, Texas and Nevada, plus a dozen more states and territories that hold caucuses to award delegates), where more consistently liberal Democrats rallied to his cause.

The comparatively more moderate Hillary Clinton, by contrast, tended to perform much better in primaries. These differences were especially obvious in Texas, which used both primaries and caucuses, held on the same day, to select delegates to the Democratic convention. Hillary Clinton won the primary vote in the morning, but Barack Obama prevailed in the caucus held that same evening.

Our research explored how voters felt about primaries or caucuses by randomly assigning people to read about the basics of how a primary or a caucus works. We then asked them to think about the next time their party would nominate a candidate and evaluate the process they had just read about, including their willingness to participate in it.

We found that those who had taken part in a caucus in the past were strongly supportive of the caucus system, but almost everyone else had concerns. They worried that caucuses favored special interests or would not be fair. And when asked whether or not they would like to participate, again we found that primaries attracted a more moderate set of voters, relative to the caucuses.

pattern. What does all of this mean for this year's election? We expect the caucuses will be friendly territory for the most ideological candidates -- Bernie Sanders on the left and Ted Cruz on the right. Drawing on the CrowdPac data on legislative record, campaign positions and donors, Seth Masket describes Sanders as the "most liberal" and also notes Cruz's clear "extremism." Others have noted the same

Though other factors will matter, too, we expect Sanders and Cruz to benefit from the difference between a caucus and a primary in states like Iowa. (Of course, even though New Hampshire holds a primary rather than a caucus, Sanders may have an advantage there because he hails from the neighboring state of Vermont.)

For a candidate like Donald Trump, who is himself less ideologically consistent and whose support comes in part from people who have not participated in the nomination process in the past, the challenge of persuading past caucusgoers to support him or convincing people to go to caucus meetings for the first time may be great.

While it is certainly possible that a nontraditional candidate like Trump will excite new voters to participate, our research tells us that it will be an uphill battle.

Candidates like Cruz and Sanders will be on their "home turf" at the caucus meetings because those meetings are likely to be filled with voters especially receptive to their more ideological messages. The single best way to change an election is to change who participates, and our choices about electoral processes -- caucuses or primaries -- do just that.