The Party Decides,” which was published in 2008, 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." Why are endorsements so important? One reason is that they have a direct effect on voters who trust the judgment of governors and members of Congress from their party. Another is that endorsements serve as a signal to other party elites about who is acceptable and who isn't. The bottom line of their analysis is this: those who are ultimately nominated typically represent the consensus choice of influential members of the party, and rank-and-file voters serve mostly to vet and validate candidates in close elections. A bit discouraging, but there you are.
In light of this study, FiveThirtyEight is keeping a running tally of endorsements ("The Endorsement Primary"). It is updated daily, so if you care, it might be the place to turn rather than the various polls that will be released with increasing frequency between now and next June. If you're wondering, Donald Trump has yet to receive his first endorsement.
Note: The above graphic is from the FiveThirtyEight website.