In the book “The Party Decides” (2008), the most comprehensive study of the invisible primary, the political scientists Marty Cohen, David Karol, Hans Noel and John Zaller evaluated data on endorsements made in presidential nomination contests between 1980 and 2004 and found that “early endorsements in the invisible primary are the most important cause of candidate success in the state primaries and caucuses.”

Endorsements aren’t a foolproof predictor. In 2008 , more Democrats initially endorsed Hillary Clinton than Barack Obama (although Obama had some support). Still, a steady flow of endorsements for Obama after his early successes in states like Iowa and South Carolina helped to signal that he was an acceptable choice among party elites and presaged his success in other states.

In the 2016 election cycle, the Democratic and Republican races look very different. Hillary Clinton had a commanding lead in endorsements even before launching her campaign — to an extent that’s unprecedented for a non-incumbent Democrat. But Republican insiders have been slow to rally behind a candidate.

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