too close to call

Resulting in too narrow a margin to make a decision, as in That ball didn't miss by much but it was too close to call, or The election was too close to call, so they decided to have a runoff. This expression comes from sports, where call has signified “a judgment” since the mid-1600s. In the 1960s it began to be applied to pre-election polls and then to the outcome of elections.