The Democratic primary race is just about over. Joe Biden's projected victories in states including Michigan, Mississippi, and Missouri on Tuesday may not have officially clinched the nomination, but the former vice president's success looks increasingly likelyvery nearly inevitable. The odds of a contested convention, which could have delivered a win to Sen. Bernie Sanders (I-Vt.) had more candidates remained in the race, are now vanishingly low; and anyway, the Democratic Party's superdelegate system means that in this two-man contest, Sanders would need to win the first ballot at the convention if he were to win at all. Next week, Biden is forecast to take majorities in three more high-delegate states (Florida, Illinois, and Ohio), enough to foreclose any last hope of a Sanders comeback.

But how? How did Biden emerge triumphant after months of uninspiring debate performances, middling fundraising numbers, minimal ground game, gaffes on gaffes on outright tall tales, and a long record of political positions now outré among much of the Democratic base?

Proffering a unifying theory of millions of people's voting choices is a fool's errand. But if I had to name a single leading cause for Biden's dominance, it's that his campaign only asked voters to answer one question. Every other candidate in the race asked two.

To support any non-Biden contender, Democratic primary voters had to say yes twice:

1. Do you want President Trump out of office?
2. Do you support this candidate and their agenda?

To support Biden, they only had to say yes once:

1. Do you want President Trump out of office?

The difference here isn't that Biden has no personality or agenda for voters to consider and accept. It's just they already did that. It's easy to feel like you made your assessment and decision about Biden years ago, in 2008 or 2012, and among Democrats, that decision was broadly affirmative.

There are real policy differences between Biden and former President Barack Obama, but Biden has emphasized the continuity his election would represent. Thus, as a person and policymaker, Biden doesn't seem like a new choice. He's already passed muster. Most Democratic voters have already determined they're fine with seeing him in the White House. They accepted the premise of Biden's leadership 12 years ago, and 12 years is a long time for a mental habit to solidify.

Confirmation bias is enormously powerful — and useful! It helps us sort through information overload, which is handy when you have to pick from among two dozen Democratic presidential candidates, each with long lists of policy ideas. But it also means Biden's campaigning can be lackadaisical and messy because the new information generated by his failures on the trail will never break through to the many, many voters who already assigned him a positive association more than a decade ago. Their brains, entirely normally and understandably, have no interest in making the same decision twice.

There are lots of reasons Biden is an enormously unpopular choice among younger voters, but this lack of pre-established acceptability is undoubtedly part of it. If you were too young to vote in 2008 or 2012, you probably didn't make a firm decision about the veep candidate. You didn't cement a strong opinion of Biden, and that enables you to notice and be influenced by new data about him in 2020.

This luxury of low-consequence campaigning is available to Biden alone purely because no other candidate in the race was Obama's vice president. No other candidate won the basic approval of a majority of their party eight or 12 years ago. No other candidate benefits from anything like the same confirmation bias. And no other candidate has the history to offer to undo the entire Trump presidency.

Moreover, for long-lasting Biden rivals like Sanders and Sen. Elizabeth Warren (D-Mass.) in particular, that second question was huge. To be sure, for many voters their agendas were incredibly appealing. That's precisely why Sanders and Warren lasted so long: Their messages resonated with a lot of people. But they were nevertheless big and complicated messages which required voters to sift through a ton of new information.

Backing Biden doesn't have that requirement. So while his competitors pitch grand ideas to transform our country, he can simply say, "Hey, what we were doing before was pretty okay. Wanna go back to that?" And from South Carolina's primary onward, the results suggest the bulk of Democratic voters are content to answer that single question "yes."

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