Parties, then, became the stewards of American democracy. Because they select our presidential candidates, parties have the ability—and, we would add, the responsibility—to keep dangerous figures out of the White House. They must, therefore, strike a balance between two roles: a democratic role, in which they choose the candidates that best represent the party’s voters; and what political scientist James Ceaser calls a “filtration” role, in which they screen out those who pose a threat to democracy or are otherwise unfit to hold office.
