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How much is Nikki Haley endorsement of Marco Rubio worth?

Perhaps more than you think. In fact, we can put a number on it. 

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    South Carolina governor Nikki Haley will endorse Republican presidential candidate Marco Rubio.
    Randall Hill and Chris Keane/Reuters
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It was leaked this afternoon that South Carolina Gov. Nikki Haley will endorse Sen. Marco Rubio for president Wednesday evening. Questions about Governor Haley’s endorsement have persisted for months. Until Wednesday, the only reliable information was that she would not be endorsing Donald Trump.

Needless to say, Haley’s endorsement is welcome news for Senator Rubio’s campaign as voters head to the polls on Saturday for the GOP primary. But just how much is Haley’s endorsement worth?

According my data, Haley’s endorsement should net Rubio an additional 4 percent on Saturday (editor's note: "4 percent" refers to an increase of 4 percentage points). But first, a little background.

Perhaps the most contentious issue among election forecasters this cycle is the extent to which party elites shape presidential nomination contest outcomes. Although primaries and caucuses give voters the power to decide who wins the nomination, and even though parties are hardly monolithic teams that act in perfect harmony, the conventional wisdom is that party elites do indeed have considerable power over who wins the nomination.

In large part this view stems from a book appropriately titled “The Party Decides.” Written by political scientists Marty Cohen, David Karol, Hans Noel, and John Zaller, the book proposes that one of the key tools used by party elites is the power of their endorsement. It is a testament to Cohen et al.’s work that Nate Silver – the most influential election prognosticator – has written extensively about the "Party Decides" thesis and has even catalogued endorsement data this cycle. Mr. Silver calls it the “endorsement primary.”

Although most political observers accept the general wisdom of the “The Party Decides,” the contentiousness surrounding this book concerns how (if at all) Mr. Trump fits into the thesis. Given that most Republican elites are apprehensive (to put it mildly) toward a Trump nomination, and Trump has secured so few endorsements, there are questions about the book’s predictive power in 2016. For a more thorough discussion of these issues, see Nate Cohen and Nate Silver.

Back to Nikki Haley and Marco Rubio. If endorsements do indeed affect who wins the nomination, it should be possible to quantify just how much Haley’s endorsement is worth.

I have been working on a project with two College of Charleston students that examines the factors that correlate with success (or failure) in South Carolina’s nomination contests. Our data cover the period from 1988 to 2012, giving us an even balance of five contested Democratic elections and five contested Republican elections.

Our primary predictors of a candidate’s vote share include their performance in New Hampshire and Iowa, how much media attention the candidate received, a candidate’s demographic characteristics, and the volume of their endorsements by state party officials.

We plan to release the model’s full predictions Friday evening and discuss more fully what factors have shaped South Carolina’s election results from 1988 to 2012. Spoiler alert: endorsements are indeed an important factor in the model.

But just how much does a governor’s endorsement matter.

According to the model, from 1988 to 2012 the governor’s endorsement has netted the endorsed candidate an additional 4 percent of the vote.

Clearly, 4 percent is not enough to vault Rubio past Trump (at least, not on its own). According to most polls, Trump has a 19 point lead over Rubio (35 percent to 16 percent). However, a 4 percent improvement in his vote share would put Rubio in second place ahead of Sen. Ted Cruz (who is polling around 18 percent).

Going forward, a second place finish would be of considerable value for Rubio’s campaign. A third place finish, however, could dim Rubio’s long term prospects.

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