Third-party candidates have had a lot of buzz this election season. Will Gary Johnson of the Libertarian Party make the presidential debates? Will he and Jill Stein of the Green Party siphon voters from Donald Trump or Hillary Clinton? Will a strong third-party vote spoil the election, as many think Ralph Nader’s Green Party bid did in 2000?

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.

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. ...