There is ample evidence that winning the party line, the preferential ballot placement that comes to candidates who win county party endorsements, gives a candidate for state or federal office an advantage.
In a recent study, Rutgers University professor Julia Sass Rubin presented data on how candidates benefit from the line, a ballot arrangement that is unique to New Jersey. This ballot arrangement, currently the subject of a federal lawsuit, is back in the news after Tammy Murphy, the wife of Gov. Phil Murphy, won the backing of the Democratic parties in the five counties with the highest number of registered Democrats within days of her announcement last month that she was running for the U.S. Senate seat currently held by embattled incumbent Democrat Bob Menendez.
Rubin looked at the results of all state legislative primaries over the last two decades, in which more than 1,000 incumbents sought reelection and 227 faced challengers. In cases where the incumbents had the party line in all the counties in which they were running, the incumbents won almost 99% of the time — only three lost. By contrast, more than half the incumbents who did not have party backing in at least one county lost.
The graphics in the article break down how many incumbents had the line in all the counties covered by their districts, how many lost the line in at least one county and whether the incumbents won.