How Will Undecided Voters Affect the Presidential Election?

With less than a month to go before the 2024 election, the polls continue to show an extremely tight race. FiveThirtyEight’s model currently shows Harris leading by an average of 2.6 points nationally, but such a small margin offers little certainty in predicting the winner. The vast majority of Americans have apparently already decided who they will vote for in November, with recent national polls suggesting no more than 5% of likely voters are unsure of who they will vote for. 

The tight race has turned the news media’s attention to focus on people who say they are still undecided on how they will vote.

Undecided voters can be challenging to study since they typically make up such a small share of the electorate. In 2016, voters who were still undecided in October made up about 8% of voters. But in 2020, that figure dropped to just 4%. Fortunately, the Cooperative Election Study (CES) has interviewed at least 60,000 Americans during each of the past several election cycles, giving us a sample of several thousand undecided voters in each election. 

Here’s what we’ve found by analyzing undecided voters over the past few election cycles: Their influence on the final outcome is often modest at best. But the margins in this year’s election are razor-thin, particularly in pivotal swing states. That means even a small percentage of undecided voters could potentially decide who occupies the White House for the next four years.

Will Undecided Americans Actually Vote? 

One critical point to remember is that undecided potential voters may not be particularly engaged in politics. Only 29% of undecided respondents in the 2020 CES could identify which party controlled the U.S. Senate (compared to 69% of those who had decided how they would vote). About two-thirds of undecided Americans did not identify with or even lean towards either party. Of those who expressed a vote choice, just 16% identified as independent.

Because people who are still undecided in October tend to be largely disengaged from politics, most end up staying home and not voting at all. We know this by matching the 2020 CES respondents to the voter file. Just one-in-four undecided respondents ended up voting. And even when we limit the analysis to undecided respondents who told us that they would definitely vote, we still find that about 40% did not ultimately cast a ballot. 

This high level of non-voting among undecided people suggests that pre-election polls may overestimate the potential impact of undecided voters on election outcomes. This is one reason why undecided “voters” often have limited influence—many are not voters at all.

Will They Vote the Same as They Have Before? 

One image that you might have of the undecided voters who do ultimately cast a ballot is that they tend to be swing or split-ticket voters. Swing voters are people who vote for a different party for president than they did in the previous election. Split-ticket voters, on the other hand, choose candidates from different parties for various offices on the ballot in any given election. For instance, they might vote for a Democrat for governor and senator but a Republican for president.

Here’s another big takeaway—undecided voters only rarely act as swing or split-ticket voters. In 2020, only about 11% of undecided voters were also swing voters. This is a higher rate of swing voting than we see among all voters—just 3% of the full electorate were swing voters in 2020. But these numbers say that the vast majority of undecided voters either didn’t vote in the previous election, or voted for the same party in 2016 as they did in 2020. 

Undecided voters are more likely to split their tickets than other voters, we found. One out of five undecided voters voted for a Republican for one office and a Democrat for another office in 2020, compared to just 5% split ticket voting among the rest of the electorate. But even so, that means that about 80% of undecided voters still voted for a straight ticket in 2020. 

This reveals a striking reality—the vast majority of undecided voters neither split their tickets nor deviated from their previous voting patterns. While they may still not be ready to state their presidential candidate choice, most undecided voters will likely vote as they have in the past, and as they will for other races on the ballot. 

How Undecided Are These Voters?

What happens to the undecided people who do ultimately vote? The graph below shows the ultimate vote choices of undecided voters over the past several presidential elections. In 2008 and 2012, undecided voters split their votes evenly between Barack Obama and the Republican nominee. That means their votes did little to move the pre-election margin. However, over the past two election cycles, undecided voters have clearly favored Donald Trump. In 2016 and 2020, Trump won undecided voters by about a 20-point margin, as the figure below shows. 

While it is hard to know for certain why this gap appeared during the past two elections, the most obvious common thread is Trump appearing on the ballot as the Republican nominee. Many Republicans were reluctant to vote for Trump. As a result, they may have remained undecided when surveyed in the lead-up to the elections. 

Many of these reluctant Republicans ultimately voted for Trump in 2016 and 2020. Others may have voted for third-party candidates, as evidenced by the increase in undecided voters supporting a candidate outside the two major parties in 2016 and 2020. 

Logically, Trump’s name on the ballot may have had the opposite effect on people who lean Democratic, cementing their support for the Democratic nominee long before they would have made up their minds if the race included a more traditional Republican. Indeed, Democrats made up a significantly higher share of undecided voters than Republicans in 2008 and 2012, but that flipped in 2016 and 2020 when more undecided voters were Republicans than Democrats.

Will They Matter? 

Will America’s undecided voters swing the 2024 election? It would take a very close election for the undecided vote to matter. Imagine that, similar to 2020, 5% of voters are currently undecided but ultimately break for Trump by an approximately 20-point margin. This would produce a net swing of about 1 percentage point towards Trump in the final vote tally. 

In an election as close as the polls are predicting, even a small shift could be decisive, especially in key swing states. Take Pennsylvania, for example—a state that may very well be the key swing state in 2024. FiveThirtyEight currently reports the polling average in that state as 0.8 points towards Harris. Thus, if undecided voters generated the same 1-point swing towards Trump that they did in 2020 and 2016, Pennsylvania could flip red—tipping its crucial electoral votes to Trump.

Still, the actual impact of undecided voters in this election remains uncertain. This year, the lesser-known candidate is Harris, and some undecided voters may still be making their minds up about her. And recent polling suggests that likely voters who are currently undecided for the 2024 election are just about evenly split between those who voted for Biden and those who voted for Trump in 2020. We know that very few undecided voters are swing voters, and they are, therefore, likely to vote as they did previously. So this suggests that this group may break more evenly than it has in the past two elections. 

Ultimately, undecided voters tend to matter only in the closest of elections, and 2024 seems poised to be just such an election. 

This story was first published in the publication Good Authority; read the original here.

Brian Schaffner is the Newhouse Professor of Civic Studies in the Department of Political Science and Tisch College. He is also a co-director for the Cooperative Election Study. 

Caroline Soler, A25, is a research associate for the Cooperative Election Study.