APRIL 5 Wisconsin 86 38/38 48/48
APRIL 9 Wyoming 14 7/5 7/9
APRIL 19 New York 247 139/122 108/125
APRIL 26 Connecticut 55 28/27 27/28
Delaware 21 12/11 9/10
Maryland 95 60/53 35/42
Pennsylvania 189 106/93 83/96
Rhode Island 24 11/11 13/13
MAY 3 Indiana 83 39/39 44/44
MAY 7 Guam 7 4/3.5 3/3.5
MAY 10 West Virginia 29 11/12 18/17
MAY 17 Kentucky 55 28/27 27/28
Oregon 61 26/24 35/37
JUNE 4 Virgin Islands 7 6/3.5 1/3.5

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.”