The 2016 Democratic and Republican presidential nomination campaigns are very different from one another. One race features a front-runner without a strong rival for the nomination. The other includes more major candidates than pollsters will list in their surveys. My new book shows that these campaigns represent distinct paradigms with different patterns of competition and coalition formation, and differences in who makes the critical decisions that nominate presidential candidates. When a dominant front-runner emerges during the invisible primary, there is less competition during the primaries and the critical decisions are made by party insiders and activists before the voters cast their ballots. When party insiders don’t coalesce strongly behind a front-runner during the invisible primary, the critical decisions are made by caucus and primary voters who get to choose among several viable candidates.

It is well known that the invisible primary — the year or so before the caucuses and primaries — plays an important part of the nomination process. One study characterizes the invisible primary as a long national discussion among party elites, groups, and activists who try to coordinate among themselves to anoint a presidential nominee even before voting begins in the caucuses and primaries. If enough party elites rally behind a front-runner, that candidate can build a sufficient lead in money, organization, and popular support to prevail in the caucuses and primaries.

That’s how the 2016 Democratic presidential nomination is unfolding.  Hillary Clinton has a host of endorsements by party elites and activists, a growing fundraising operation, and a strong lead in presidential opinion polls. As a result, some would-be candidates are staying out of the race, leaving Clinton to run virtually unopposed. That makes the front-runner’s path to the nomination much easier. In nomination campaigns like this, a rival or two may emerge — but caucuses and primary voters generally support the candidate backed by party elites and group leaders. The main question is whether the front-runner will stumble enough to blow the nomination, which hasn’t happened since Gary Hart’s fall from grace during the invisible primary of 1988.

The 2016 Republican race, in contrast, shows some serious competition with a lot of candidates, but none with a meaningful plurality of support in opinion polls. As a result, there is little to deter even more politicians like Lindsay Graham from entering the race. With a number of potentially appealing candidates in the race, Republican insiders are undecided and divided about who they will support. Most Republican elites are standing on the sidelines, waiting to see who catches fire before endorsing a candidate. As a result, rank-and-file Republicans lack clear signals about who to support. Such nomination campaigns tend to remain competitive at least through the Iowa caucuses and the New Hampshire primary, when party activists and a broader population of party identifiers begin to settle the matter. These nominations tend to hinge more on campaign momentum gained or lost during the primaries, rather than the machinations of party insiders during the invisible primary.

Since 1972, about two-thirds of the presidential nominations have been largely settled by the end of the invisible primary — but one-third have not. Consider the Democratic nominations of 1972, 1976, 1988, 1992, and 2008 and the Republican nominations of 2008 and 2012. As the figure below indicates, competitive nomination races have had a larger number of viable candidates in the primaries, which give these voters a meaningful choice among candidates. There are fewer viable candidates in the primaries when party insiders and party identifiers unify behind a front-runner during the invisible primary. Which of these scenarios emerges in a given election year depends largely on the unity and stability of the political party coalitions and on which candidates decide enter the race.

The unity of each party’s coalition varies over time. It is easier for party elites to unify behind a front-runner when the party coalition is stable than when the coalition is divided. For instance, Democratic nominations in the 1970s and 1980s were competitive in part because the Democratic Party was more divided as William Mayer has shown. Democrats appear to be more unified in the past twenty years and their presidential nominations have become less competitive. In contrast, Republican nominations from 1980 through 2004 were not competitive. This owes in part to the closing of Republican Party internal divisions during the Ronald Reagan years. Republican presidential nominations are becoming more competitive as party divisions emerged in the aftermath of the Bush years. While Republicans are unified around broad themes of lower deficits, less government, and lower taxes, they differ over policies and priorities. Consider the differences between social conservatives and libertarians, supply-siders and deficit hawks, and between neo-isolationists and hardliners. Reconciling these differences is particularly challenging in a race in which several candidates try to establish themselves by carving out a niche of support from groups in the broader party coalition.

How much party insiders coalesce behind a front-runner also depends on who enters the race. Party insiders unify much earlier in the invisible primary when their nomination race features a candidate who had a majority lead in public opinion polls taken before the mid-term congressional elections. Hillary Clinton is such a candidate. Many progressive Democrats are not fans of Hillary Clinton, but they will have little choice if Elizabeth Warren does not enter the race. Warren knows that it is a gamble to enter the race against a candidate who already has a substantial bloc of support among party insiders, donors, and identifiers. Democratic nominations have been competitive whenever they have had an expected front-runner who declined to enter the race, such as Ted Kennedy in 1976 or Mario Cuomo in 1992, or who dropped out, as happened with Gary Hart in 1988.

In comparison, the 2016 Republican race is historic in that Republicans do not have a similarly situated candidate. In almost every presidential nomination race since the 1970s, the Republicans have had a candidate with a large plurality or even a majority in pre-midterm, national opinion polls. Without that clear front-runner, most Republican insiders are waiting to see which candidate gets constituents excited. Could any of the GOP candidates actually win the invisible primary this year? That’s an open question. Nate Cohn wrote that it would be Bush, or no one. Nate Silver and Harry Enten gathered the endorsement data to show just how few endorsements have been made. Campaigns like this can—and often do—take a long time to sort out and typically remain competitive at least through the early caucuses and primaries, when voters begin to separate the winners and losers. One potential caveat: Republican elites may still unify around Jeb Bush who has yet to declare. John Sides and Lynn Vavreck have shown that the choice of party insiders still has an edge in the caucuses and primaries even if most party elites refrain from a public commitment of support.

Wayne Steger is Professor of Political Science at DePaul University. He specializes in the study of American presidential nominations, with a focus on party elite behavior and coalition formation.