This extra bonus to Republican candidates for a win has some important effects. If a Democratic candidate can keep the vote margin close, support in future state primaries does not depend on whether they won previous primaries. The result is that Democratic candidates can receive roughly the same level of support from Democratic voters, even if they do not rack up state wins. This has the effect of encouraging Democratic candidates to stay in the race. Since 1980, Democrats have been more likely to drop out early in the race, but if they make it through the first three months of the primary campaign, they become much less likely to quit than Republicans. Any given Republican candidate is about 20 percent more likely to quit the race once the campaign is about two-thirds of the way through (about mid- to late May in most recent primary campaigns) as compared to Democrats. As a result, over the course of the primary, Democratic candidates do not consolidate the primary vote as fast as Republican candidates do, which drags out the process.

To measure how difficult it is for Democrats to consolidate the primary vote, I examined the average Election Day vote share received by the Republican and Democratic winners since 1980 across the primary calendar. (One important caveat: I only examined competitive primaries, with no incumbent president or vice-president running.) In 2016, despite the number of candidates in the Republican race, the lack of endorsements or fund-raising by Trump, and the outright opposition to him by much of the party elite, his average vote share steadily increased across time, reaching 50 percent on the 12th primary contest day (April 19th, which was New York’s primary) and never dipping below that subsequently. Clinton, on the other hand, won over 50 percent of the vote on February 23rd, then 73 percent of vote on February 27th, but three primary elections later received only 36 percent of the vote. Clinton received less than 50 percent of the vote as late as June 7th, and never managed to decisively pull away from Sanders. In fact, only Bill Clinton in 1992 was able to consolidate the Democratic vote over time in a way similar to every Republican candidate with the exception of Reagan in 1980.