What to Make of the ‘Zombie Vote’ Against Donald Trump

Even after Nikki Haley dropped out of the Republican presidential primary, effectively handing the party’s nomination to former President Donald J. Trump, nearly 20 percent of G.O.P. primary voters have cast ballots for someone other than Mr. Trump. The Pennsylvania primary on Tuesday, where Ms. Haley won more than 16 percent, was just the latest example.

These anti-Trump votes have been closely watched, particularly in light of the unusually high number of votes for “uncommitted” and candidates other than President Biden in this year’s Democratic primary.

Ballots cast for candidates who have suspended their campaigns are sometimes called zombie votes. This phenomenon is hardly new.

In fact, a review of contested primaries since 2000 reveals that sizable shares of the electorate routinely chose someone other than the eventual nominee, even after all other serious contenders had dropped out.

In 2020, Senator Bernie Sanders of Vermont withdrew from the Democratic primary on April 8, leaving Joseph R. Biden Jr. as the only serious candidate in the race. Still, in the weeks and months that followed, Mr. Sanders received votes. In the Pennsylvania Democratic primary on June 2, 2020, for example, more than 20 percent of voters chose someone other than Mr. Biden, including 18 percent who selected Mr. Sanders.

The zombie vote in this year’s Republican primary has actually been low by historical standards. In Democratic and Republican primaries going back to 2000, roughly a quarter of voters picked a candidate other than the eventual nominee even after all the other serious contenders had exited the race.

There are many factors that lead to zombie votes. Not all of them indicate a true protest vote from party loyalists against the eventual nominee.

One factor is the rise of early voting and mail ballots. In Florida, for example, around one in three Republican voters had mailed their ballots before Ms. Haley dropped out on March 6.

However, she has continued winning a decent chunk of votes in states where nearly all voting has occurred after her departure. In the Wisconsin election on April 2, Ms. Haley won 13 percent of the vote.

The Trump campaign has argued that some of Ms. Haley’s support has come from Democrats voting in Republican contests. This may explain the zombie vote in, for example, Georgia, where any voter can vote in either presidential primary.

But the pattern has persisted even in states like Pennsylvania, Connecticut and New York where the primaries were open only to registered Republicans.

Unlike protest votes against incumbent presidents in the form of minor candidates and ballots cast for “uncommitted,” contested primaries feature big-name candidates who get wide media exposure, have clear policy differences and forge emotional connections with many voters. Perhaps the affinity some voters develop is the easiest explanation of the persistent zombie vote across so many election cycles.

We shouldn’t expect the zombie vote to go away any time soon. In the past, the share of voters still supporting candidates who had already withdrawn remained relatively consistent, even on the last day of primaries held before the party conventions. And there is no correlation between the size of the zombie vote and the eventual nominee’s chances of winning or losing in the general election.