Currently, the party balance in the United States is nearly even, roughly one-third Democratic, one-third Republican, and one-third independent, taking turnout into account. This means that to win a majority a party normally must capture at least as large a share of independents as the other party. Thus, independents constitute the marginal members of an electoral majority. We do not know nearly as much about this critical group of voters as many pundits think. The electoral movements of this poorly understood category underlie the unstable majorities of our time.