If you were hiring someone for a vitally important job, would you begin by interviewing 10 applicants at a time, hoping that the ones with the snappiest sound bites might also be most qualified for the job? Of course not. And yet that’s the bizarre logic behind this week’s Democratic presidential debates.

That abundance of candidates may seem like a good thing — democracy! — but it isn’t. As we all should have learned from the Republican debacle in 2016, the presidential nominating process is utterly unequipped to deal with such a large field. Donald Trump was nominated because his mainstream opponents could not knit their supporters into a majority, in effect ceding power to an intense minority within the party. His poll numbers barely cracked 35 percent through mid-March and still hadn’t reached 50 percent when the race effectively ended in early May.

The current Democratic candidates pose no threat to the Republic — unless one of them loses the general election. Still, the Democrats have their own unhappy experience with an unruly primary. Two years before the 1976 election, a Gallup poll listing 31 candidates for the Democratic presidential nomination did not include an obscure one-term governor named Jimmy Carter. A year later, Mr. Carter’s support among Democrats remained below 5 percent. Yet he emerged from a crowded field to win the nomination and (barely) the general election. Alas, Mr. Carter was a better candidate than president.

Cognitive psychologists tell us that human information-processing capacity is limited to seven objects, plus or minus two. But when the objects are as complex and unfamiliar as the current crop of presidential candidates, that rule of thumb is much too optimistic.