The Endorsement Primary

UPDATE (June 9): We are no longer keeping track of primary endorsements, as both Donald Trump and Hillary Clinton have clinched their respective nominations.

UPDATE (May 23): Donald Trump, who didn’t receive his first endorsement until three weeks after the Iowa caucuses, is the presumptive Republican nominee for president. Trump will enter the Republican National Convention with the fewest primary endorsements — by far — of any major party nominee since at least 1980. With Trump as the last GOP candidate remaining, any additional endorsements for him will be for the general election, not the primaries. As such, we won’t be listing those endorsements on this page.

Before any votes are cast, presidential candidates compete for the support of influential members of their party, especially elected officials like U.S. representatives, senators and governors. During the period known as the “invisible primary,” these “party elites” seek to coalesce around the candidates they find most acceptable as their party’s nominee. Over the past few decades, when these elites have reached a consensus on the best candidate, rank-and-file voters have usually followed.

Of course, not all endorsements are equally valuable. We use a simple weighting system: 10 points for governors, 5 points for U.S. senators and 1 point for U.S. representatives (there are roughly five times as many representatives as senators and 10 times as many representatives as governors).

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.”

These endorsements can serve several purposes. In some cases, they directly influence voters who trust the judgment of governors and members of Congress from their party. In other cases, endorsements serve as a signal to other party elites. “It tells others who is acceptable and who is unacceptable,” Cohen, an associate professor of political science at James Madison University, said in an e-mail to FiveThirtyEight. “This is the coordination process that we believe goes on during the invisible primary and by way of public endorsements that was formerly and more formally undertaken at the convention.”

While each election cycle is different, nomination races since 1980 have fallen into two broad categories. In some — like the Republican contest of 1996 and both the Democratic and Republican races of 2000 — party elites rallied around a candidate well before the Iowa caucuses, and these candidates went on to win their nominations with relative ease. In others, like the Democratic contests of 1988 and 2004 , there was less of an endorsement consensus, with party officials taking more of a wait-and-see approach until voters weighed in. Party elites coalesced around a candidate only after the Iowa caucuses and the New Hampshire primary.

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.

If you see an endorsement from a governor, U.S. senator or U.S. representative that we haven’t included, please email us. While endorsements from other party elites, such as local party officials in Iowa and New Hampshire, often matter as well, for the sake of simplicity, we won’t be focusing on them here.