Put another way, voters’ partisan and ideological identity as Democrats or Republicans, liberals or conservatives, drives political decisions as much or more than “distinct policy goals.”

Identity, not ideology, she writes, is crucial:

The power behind the labels “liberal” and “conservative” to predict strong preferences for the ideological in-group is based largely in the social identification with those groups, not in the organization of attitudes associated with the labels.

The same forces Mason describes can, in turn, create an ideological orthodoxy that rejects challenges from those who dissent, as Bill Clinton famously dissented by veering toward the center in his 1992 bid for the Democratic nomination.

Ideological orthodoxy may not encourage the kind of independent candidate who might be best equipped to take on Trump — an erratic but resourceful maverick.

Peter Ditto, a professor of psychological science at the University of California-Irvine, picked up this theme in an email:

At one level, people respond positively to mavericks. Americans love independence and authenticity as personality traits. We valorize people who go their own way and follow their own principles — following your conscience and blazing your own trail are iconic American characteristics.

Conversely, Ditto wrote,

perhaps the most fundamental principle of interpersonal attraction is that similarity attracts, in particular, that people like others who share their same beliefs and attitudes. This means that if someone on your side, of your tribe, becomes a maverick, endorses political positions of the other side, they will almost always disagree with you — and nobody likes that. Liberals loved John McCain when he took maverick positions against the G.O.P., but were much less fond of Joe Lieberman when he did the same.

In partisan terms, this means that “people like a maverick of the other party more than a party line guy, but like a maverick of their own party less than a party line guy.”

Biden, Ditto argued, faces just this problem:

He wants to capture the character traits of equanimity and bipartisanism, which many people in the electorate seem to have some longing for but it cuts against the grain of the progressive wing of the Democratic Party who are pushing for aggressive policy changes.

While this is “the tightrope that every modern presidential candidate has had to walk as they pivot from the primaries to the general election,” Ditto wrote, there are factors “that make things more complicated now.”

The crucial factor, according to Ditto, is that

we now live in a tribalized political world, a world where issues have become moralized, and political sides rally around symbolic political totems. In this kind of tribal environment, disagreement is sacrilege and mavericks are apostates.

In this “heavily moralized, deeply tribalized political climate,” any disagreement with

your side’s policy positions is often seen as a sign of disloyalty rather than diversity of thought, a call for censure rather than debate. This will make it tough for the eventual Democratic nominee to walk the line between holding the base and attracting the middle.

Trump is one kind of maverick. The progressive wing of the Democratic coalition is replete with its own mavericks this year, and that may or may not work in its favor. Progressives have a raft of proposals shunned by the more centrist Democratic establishment — decriminalizing border crossings, for example. In this case, the establishment pushback has been swift and angry.

The sharp exchange over border funding legislation and the use and misuse of political power between the Speaker of the House, Nancy Pelosi, and the chamber’s four most prominent left dissidents, Representatives Alexandria Ocasio-Cortez, Ilhan Omar, Ayanna Pressley and Rashida Tlaib, tells us a lot about the depth of the tension between progressives and the more moderate wing of the party.