He did so in large part because of demographics. Former South Bend, Ind., mayor Pete Buttigieg had done well with white moderates in Iowa and New Hampshire, but he proved incapable of winning black or Latino support in Nevada or South Carolina. Sen. Amy Klobuchar (D-Minn.) similarly peeled away white moderate votes from Biden in early races but had no significant minority support. Once Biden showed he could win supermajority support among blacks while holding his own with whites in South Carolina, it was clear Buttigieg and Klobuchar had no chance to win the nomination. Their quick exit and endorsements of Biden allowed them to gracefully depart the race while currying favor with the establishment to which they both belonged.
Luck played a key role as well. Biden had one last potentially viable moderate competitor: former New York mayor Mike Bloomberg. His record campaign spending had made him a contender in many Super Tuesday states, but he had the misfortune of agreeing to participate in his first primary season debates before those states voted. His disastrous first performance and a mediocre second debate left him exposed as a rich emperor with no political clothes. Biden did not shine in either outing, but neither did he prove unprepared or incapable of taking predictable and tough questions. Establishment Democrats bent on beating Sanders thus had only one viable option left: Back Biden and hope for the best.
Contrast this with the voter dynamics and circumstances of Trump’s rise. Contrary to common wisdom, Trump’s rise was not fueled by the Republican right. Sen. Ted Cruz (R-Tex.) emerged as the GOP version of Sanders — the champion of the vocal ideological minority within the party. Cruz beat Trump among “very conservative” voters in 14 states, but Cruz’s support was even more heavily tilted to the right than Sanders’s support is to the left. Trump cruised to the nomination by crushing the Texas senator in state after state among the two-thirds or so of GOP primary voters who were not “very conservative.”
Trump’s other final opponent, Ohio Gov. John Kasich, also had a narrow base of appeal. Kasich was championed by the party’s moderates but loathed by very conservative Republicans. Exit polls show that his support was as tilted heavily to the party’s left as Cruz’s was to the right. As a result, neither man’s supporters were uniformly willing to back the other to beat Trump. A March 2016 Public Policy Polling poll found that only half of Kasich’s voters would back Cruz in a one-on-one contest with Trump. Cruz’s voters were similarly divided, with slightly less than half saying they would back Kasich if he were matched against Trump one-on-one.
Luck and circumstance played roles in the 2016 Republican contest, too. Sen. Marco Rubio (R-Fla.) was the only serious contender who had significant support among all three of the party’s ideological factions. But he stumbled badly in a debate just three days before the New Hampshire primary, causing him to finish a poor fifth. He came back to finish second in South Carolina and Nevada, but that meant that he, unlike Biden, had no convincing win to rally party leaders to his cause before the GOP’s Super Tuesday. A feud with former Florida governor Jeb Bush, the favorite of many party leaders and donors, also meant that Bush would not make way for Rubio even as Bush’s hopes lay dormant. Cruz’s own feuds with party leaders also made it less likely that they would back his campaign when he became the only viable option to stop Trump.
But Biden and Trump’s rises do have one thing in common: They both secured the key party faction — blacks in Biden’s case, somewhat conservatives in Trump’s — that has backed the winner in each of their respective parties’ contests since 1992. Looks as though it’s the voters that decide, not the party.