The 2024 Republican presidential primary could be over in a month

Friday marks one month until the 2024 Iowa caucuses, the official start to the Republican presidential nominating process. But the primary has been carrying on unofficially for nearly a year now — and in some ways, it is basically already over.

According to 538's average of national polls, as of Dec. 15, former President Donald Trump leads the primary race with 61 percent. Florida Gov. Ron DeSantis and former U.N. Ambassador Nikki Haley are essentially tied for second at 12 percent and 11 percent, respectively. Trump also leads by at least 25 percentage points in Iowa, New Hampshire and South Carolina. And when you consider Trump's wide lead alongside how little time remains until voting actually begins, he now looks almost inevitable. Unless a historically unforeseen event happens, Trump is overwhelmingly likely to be his party's nominee next year.

National polls

In late August, I published an analysis of historical and contemporary polls that gave Trump a roughly 4-in-5 chance of winning the GOP presidential nomination. That article looked at how often past presidential candidates polling at 50 percent nationally (as Trump was in August) went on to win their party's nod. I also produced odds for other candidates, ranging from about a 1-in-20 chance for Haley (then at 4 percent in national polls) to a 1-in-8 shot for DeSantis (15 percent). Now, with new polls and less time for a candidate to gain (or lose) ground in the race, let's check in on the model again.

Since August, Trump has consolidated significant support from GOP voters, and that has raised his probability of winning the primary. His 61 percent in the polls now equates to a more than 9-in-10 chance of winning the nomination.* No presidential candidate in history has lost the nomination while being so far ahead in the national polls this late in the cycle. The highest-polling losing candidate was then-Sen. Hillary Clinton in 2008 — who led the national polls with slightly less than 40 percent in mid-December 2007. (And she still won 23 contests and 47 percent of delegates at the August Democratic National Convention.)

Polling leaders in December usually win the primaries

538′s retroactive average of national polls for non-incumbents running in presidential primaries from 1980 to 2020, as of Dec. 15 before the election year

Sources: Polls, Congressional Quarterly, Dave Leip’s Atlas of U.S. Presidential elections, Federal Election Commission

The polling shows a striking asymmetry in presidential primary odds. When surprises happen, it's usually when upstart candidates stage comebacks. But clear leaders rarely lose. Every candidate with at least 40 percent in national polls in mid-December has gone on to win their party's nomination. Three candidates polling below 15 percent — former Massachusetts Gov. Michael Dukakis in 1988, then-Arkansas Gov. Bill Clinton in 1992 and former Sen. John McCain in 2008 — did go on to win. But those were wide-open races; in each cycle, no other candidate was above 25 percent in the polls in December. Today, Trump commands more than double that support, and that means there are fewer voters around to change their minds.

Here's another way to think about it: Over the course of all presidential primary contests since 1980, the average candidate saw their national polls move by about 17 percentage points total between their first set of polls and the end of the contest (or when they dropped out). As of Dec. 15, 2023, the average candidate has already seen their polls move by roughly 8 points. That means the average candidate has about 9 points of movement left. Even if Haley or DeSantis gained double that — what we'd expect to happen roughly one out of 20 times if we could re-run the primary over and over— they would only be polling at around 30 percent. And even if all those votes came from Trump, he would still be ahead of them by about 13 points. In fact, even if all remaining GOP primary voters chose one single non-Trump alternative, he would still lead nearly two voters to one.

Of course, for any shifts in the race to really matter, it would have to happen before Americans start voting. That brings me to …

Iowa

One obvious catalyst for change in the polls is a surprise in Iowa, the first state to vote in every presidential nominating contest since 1972. If a candidate beats expectations there, they often enjoy a bounce in later contests. Think then-Sen. Barack Obama in 2008 (he beat his polling average on RealClearPolitics.com by 7 points), former Sen. Rick Santorum in 2012 (8 points) or Sen. Marco Rubio in 2016 (6 points). Of course, only one of those men went on to win his party's nomination for the presidency, so the bounce is no guarantee of victory.

However, it is doubtful that an overperformance of 5-10 points would suffice to boost Haley or DeSantis significantly. If the Iowa caucuses were held today, polls indicate Trump would likely win by about 27 points. A 17-point win would be similarly formidable. In the cases of Obama and Santorum, their overperformance resulted in surprise wins, not just beating expectations. So a Trump alternative also needs to gain significant ground before Iowa to stage a proper comeback.

Here, too, there is bad news for the #NeverTrump crowd. In the month leading up to both parties' Iowa caucuses since 2008, the average candidate's support in Iowa polls has only changed by about 4 points.

Iowa polls usually change little after December

Each presidential candidate’s RealClearPolitics average of the Iowa caucuses 30 days before the election and on election day, since 2008

Herman Cain dropped out of the 2012 Republican primary before the Iowa caucuses.

Source: RealClearPolitics

Only in one case did a candidate gain double digits (Santorum in the 2012 GOP primary) — and that shake-up was driven by one candidate (former House Speaker Newt Gingrich) losing significant ground and another (businessman Herman Cain) dropping out of the race entirely. Absent a high-profile dropout ahead of Iowa this year, it's hard to see where any current candidate could gain such momentum.

There is still a chance that the polls are wrong or that history does not serve as a guide to the 2024 primary. Unknown variables range from whether Trump will be convicted of a felony before officially being nominated in July, to how long his leading opponents will stay in the race and whether Republican voters start to worry that Trump might be "unelectable" in a general election.

But the impacts of these events feel tiny compared to Trump's current commanding lead. Four months ago, he looked like an obvious leader, but not a foregone conclusion. Now, the hopes for his competition lie on a once-in-a-generation political comeback. Do any of them have what it takes?

CORRECTION (Dec. 15, 2023, 4:28 p.m.): A previous version of this article stated that multiple presidential candidates had gained double digits in Iowa in the final month before the caucuses. While two presidential candidates have seen their polls change by double digits in Iowa in the final month, only one saw a double-digit gain.

Footnotes

*I have modified the model since August. Last time we used a logistic regression, which treats events as individual outcomes independent of other outcomes. Of course, in a primary, only one candidate can win, so it's appropriate to use a multinomial model instead. Based on feedback from Cory McCartan, an assistant professor of data science at New York University, I performed this model using something called the "poisson trick" for using one logistic model to make predictions on multinomial outcomes. McCartan has published several model options on his Github page, which I adapted for this article.

The 2024 Republican presidential primary could be over in a month originally appeared on abcnews.go.com