Most of these discussions have focused on the presidential race. But that misses something intriguing. Third-party candidacies in congressional races have been climbing steadily for decades.

Over the past 100 years, U.S. third parties disappeared — and then returned

Relatively small or regional parties are influential in many democracies. But the United States has for a century had one of the world’s most stable two-party systems.

AD

That wasn’t so throughout most of the nation’s history. Until the 1920s, various socialist, populist and abolitionist parties influenced U.S. politics and policies in significant ways, adding to the national agenda a host of issues including the abolition of slavery and the eight-hour workday.

But by the 1950s, third parties had all but disappeared, except for occasional segregationist candidates for president. Most U.S. political scientists and commentators have assumed that’s the end of it, and that third parties will never again be prominent.

But is that necessarily so? The evidence below suggests that third parties are rebounding. As the graph shows, 50 years ago, third-party candidates made it onto the ballot in fewer than 1 in 10 races for the U.S. House of Representatives. That number has risen fairly steadily since. Over the past 20 years, well over half of all U.S. House districts have had a third-party candidate on the ballot.

Quantity doesn’t necessarily mean popularity. More third-party candidates have been running for office — but they’re still pulling in very small percentages of the vote. That’s true even in years when third-party presidential candidates have fared comparatively well. As the chart below shows, from the Civil War to the 1910s, the share of the vote cast for third-party congressional candidates regularly topped 10 percent. Since 1920, third-party congressional candidates have received more than 3 percent of the vote only twice: in 2000 (3.33%) and 2006 (3.01%).

Here are the usual explanations for why third parties disappeared. They’re wrong.

AD

AD

Scholars and pundits offer several possible explanations for this. In a 2014 study, we found that most explanations are either wrong or overstated.

1. Duverger’s Law

Duverger’s Law argues that when democracies elect one member of the legislature per district, and when the candidate with the most votes wins the district, the result is a stable two-party system. Compare this with other countries’ systems, such as proportional representation, in which each party wins a proportion of the overall legislative seats that matches its proportion of the overall vote – so that if a third party got 15 percent of the vote, it would get 15 percent of the legislative seats.

It is true that democracies with single-member, winner-take-all districts have fewer parties than other systems. And yet the United States is the only true two-party system among the world’s major democracies of this type. Consider that Britain, Canada and India all have vibrant multiparty systems in which third parties typically receive at least one-fifth of the parliamentary vote.

AD

AD

2. State ballot access laws

Some have argued that when states have laws that make it hard to get on the ballot, that discourages third parties. It’s not so. We found no relationship between state election laws and third-party access. Over the past decade, third parties have been getting onto the ballot at roughly the same rate as in the 1920s, even though laws have made that more difficult. While parties must often solicit thousands, or even tens of thousands, of signatures to get a candidate listed, the Greens, Libertarians and others have managed to do it, except in a few extreme cases where state rules do make it almost impossible.

3. Party Primaries

AD

Some have argued that primary elections have undermined third parties. How? Once, party bosses decided who got the nomination – and the only option an anti-establishment candidate had was to run on a different ticket. Primaries mean that outsider candidates can run as Democrats or Republicans, as tea party candidates did in 2010, 2012 and 2014, and as both Donald Trump and Bernie Sanders did in 2016. So who needs third parties?

But we found no evidence that primaries affected third parties. Wisconsin and Minnesota were among the two earliest states to enact primaries near the turn of the 20th century – and they still had strong third parties into the 1940s.

AD

So what are some other possible explanations for the fall and return of third parties?

AD

1. Partisan polarization

In the late 19th and early 20th centuries, the parties were very distinct ideologically – and their memberships were as well. Political scientists call that partisan polarization, in which people and parties are highly sorted by opposing stances on a range of policy issues. Those differences between the parties and their followers were very low in the middle of the 20th century – and are high again today.

Polarization and third-party candidates have fallen and risen in sync. Perhaps the parties’ increasing cohesiveness leaves less room for party “outliers” – who then run for office outside the system.

2. Economic inequality

AD

Here’s what else rose and fell in sync with polarization and third parties: economic inequality. It was high before and into the 1930s, fell until the 1960s, and has risen again to levels not seen since before the Great Depression. Rising inequality may increase frustration with the established order – and the chance that voters will reject the major parties.

AD

3. End of the Cold War

Fear of communism once squelched many forms of leftist dissent. The United States had a comparatively strong socialist party in the 1920s, but by the 1950s, several states denied leftist parties a place on the ballot and disqualified communists from public employment.

AD

However, by the 1970s, McCarthy-era repression had largely waned. Leftist parties slowly reemerged, adding to third-party expansion on one side of the political spectrum. Left-leaning parties aren’t yet as numerous and active as right-leaning parties, but they are increasingly active. For example, the Working Families Party, which was formed in New York in 1998, has been expanding to other states. In 2014, it also ran House candidates in Connecticut, Oregon and South Carolina.

4. Communication technology has made it easier for third parties to organize and reach out

AD

During the 19th century, political parties often relied on the efforts of local volunteers rather than paid strategists. By the 1970s, communication technology had transformed campaigns, as parties could use television and radio to reach voters. This produced a decisive advantage for major parties, as this modern approach required money — to run ads, mail campaign literature, and so on. In short, major parties could reach more voters.

AD

That has changed with the rise of the Internet. Younger voters, who are already more open to socialist or redistributive movements than previous generations were, can be easily reached via social media. Some may support third-party candidates.

So what are the prospects for third parties in 2016 and beyond?

This November, third-party presidential candidates are likely to attract more voters than in any presidential election since 1996, when H. Ross Perot received more than 8 million votes on the Reform Party ticket. But the presidency isn’t the United States’ only political race. Keep an eye open; third parties may be growing in down-ballot races as well.

Matthew Dean Hindman is assistant professor of political science at the University of Tulsa.