Dating back to the beginning of the modern presidential primary era in 1976, there has been one consistent thing about almost every primary season. By looking at one particular factor, you could tell early on who was going to be the nominee of their respective party. That is, whoever was ahead in total votes and delegates after the first five contests has won nearly every time. The sole exception was in 1992 Democratic primaries, when Bill Clinton won the nomination despite having not won any of the first five contests. His first win didn’t come until the Georgia primary, which was sixth in the schedule. However, there was no solid leader after the first five that year, with three other candidates splitting them up.
In 1976, both Gerald Ford and Jimmy Carter were the clear leaders in their respective party’s nomination fight after the first five primaries and caucuses. They both won their party's nomination. In 1980, Ronald Reagan won the majority of the first five GOP contests. He was nominated. In 1984, Walter Mondale was barely ahead of Gary Hart at that point in the Democratic nomination battle, but he won the nomination. Hart had actually won three contests to Mondale’s two, but Hart’s wins were in some very small states, so Mondale held a slim lead in total votes and pledged delegates.
In 1988, Michael Dukakis took the lead shortly after winning New Hampshire primary (the second contest), and never looked back, on his way to winning the Democratic presidential nomination. Ditto George H.W. Bush on the Republican side that same year. In 1996, after being upset by Pat Buchanan in New Hampshire, Bob Dole re-took the lead for good by winning South Carolina (the third contest) and coasted to the GOP nomination. In 2000, Al Gore swept through all of the first five contests on his way to the Democratic nomination. That same year, George W. Bush lost two the first four GOP contests to John McCain, but put him away in the fifth one (Florida).
In 2004, John Kerry won four of first five Democratic contests to get on a glide path to the nomination. In 2008, Barack Obama pulled ahead of heavy favorite Hillary Clinton by routing her in South Carolina (the third contest). She was never quite able to catch up from there, and Obama went on the take the Democratic nomination. South Carolina was also the venue in 2008 where John McCain was finally able to pull away from his Republican opponents for good. It was their fourth contest of the season.
In 2012, Mitt Romney lost two of the first three contests but took the lead for good against his GOP rivals by winning big in Nevada and Florida, the fourth and fifth contests, respectively. This year, Donald Trump, after losing the opening contest in Iowa to Ted Cruz, won the next three in a row and never again relinquished the frontrunner position on his way to becoming the presumptive Republican nominee. Also this year, Hillary Clinton beat Bernie Sanders in three of the first four contests to begin building the insurmountable head she now has in her quest for the Democratic nomination.