https://www.jstor.org/stable/20452370?seq=4

This journal article sheds light on the role that the invisible primary plays in the context of the final results of the election. They discuss the importance of momentum in presidential elections and show that candidates who receive a substantial boost of support from Iowa and/or New Hampshire at the outset of the primaries are very difficult to beat, as they are able to increasingly build momentum throughout the election period. If a candidate has success in one time period, their success in the next will be even greater and so on. They explain that what dictates a candidate’s success and begins this effect in the very first time period (ie. in Iowa/New Hampshire) is the invisible primary. It is what “sets the starting position.” They argue if a candidate enters the first primary with substantial support from the invisible primary, they will succeed at time point 1, and that success will then snowball and grow throughout the election and vice versa. Thus, the level of support in the invisible primary weighs heavily on the ultimate success of a candidate.