Consider a typical partisan election in the United States. Democrat John Smith and Republican Jane Jones are the candidates in the general election. Perhaps one or two minor parties also have nominees on the ballot, but these candidates are not expected to poll above the single digits, let alone threaten the two major candidates. There might even be an independent candidate who has gathered enough petition signatures to earn a spot on the general election ballot; but again, he is not a serious contender. Over time, given the inability of the minor parties and independent candidates to win elections, their support dwindles. Voters who might agree with them ideologically do not see the point in “throwing away” their votes on candidates with no chance of winning. Eventually, such voters may simply stay home, unwilling either to support the major parties or to cast a futile vote.
This familiar dynamic may seem inevitable in a “first-past-the-post” or “plurality” —where the candidate with the most votes wins, even if no candidate gets a majority of the votes cast—but it is not. Throughout the nineteenth century, third parties flourished in the United States; then as now, American elections featured single-member districts in which the candidate with a plurality of votes won and there were no prizes for parties that gained a significant minority of votes. What has changed is that the large majority of states have banned fusion, a common nineteenth-century practice that is unheard-of in most of the country today.