The topic of third parties is coming up, what with the empty talk of the Conservative Movement (TM) running an independent candidate, and the more realistic prospect of Bernie running as an independent (or at any rate, his supporters choosing him as a write-in candidate rather than Crooked Hillary).
Many on the progressive side wish there were more than just the two national parties -- "like they have in Sweden," or wherever else they imagine electoral utopia exists.
But America already has a multi-party system -- each of the two parties is always a coalition of several distinct factions. Before the recent disruptions of Donald Trump and Bernie Sanders, the Republicans were a coalition of the US Chamber of Commerce, the Cultural Right, and the neoconservative warhawks, while the Democrats were a coalition of the US Chamber of Commerce, the Cultural Left, and neoliberal regime-changers.
Even the Cultural Left and Right are coalitions of distinct factions who don't have anything immediately in common, and have to convince each other that there's a bigger cause uniting them all -- pro-lifers, preppers, gun nuts, apocalyptic cults, etc. on the Right, and AIDS propagators, feminazis, aggrieved racial minorities, cosplay environmentalists, etc. on the Left.
"In Sweden" (or wherever), each of these narrowly focused groups might found their own party and run their own candidates. Given how narrow their focus is, they would likely form coalition governments after the election was held.
In America, they form coalitions before the election, and each of these coalitions runs in the general election. It seems better to build the coalition first, so you can hit the ground running if you win the election, rather than cultivate relationships and ties with a coalition government after the election, wasting precious time when you're already in office.
Still, why not three major parties, each of them a coalition as now? Probably to prevent yet another level of coalition-building. With three equally strong major parties, two would probably have to join up in a super-coalition against the third. This would be another case of having to establish links, build relationships, and so on, after the election, wasting time while in office.
And why not just one party? That might work during exceptionally harmonious periods, like the Era of Good Feelings when the Democratic-Republican Party went unopposed at the national level. Otherwise, we're going to need some choice between competing interests.
The other knock against the two-party system is that it encourages ossification of whose interests are represented by the only effective parties. But that's not true either, because the composition of either party's coalition is always subject to change, or re-alignment.
The Republican coalition during the Bush Sr. and Jr. era would have looked utterly alien to the Republican coalition of the Eisenhower and Nixon era. Back then, it was the Democrats who were more established in the Deep South, and who were interventionist warhawks. Likewise the Democrat coalition of the Clinton and Obama era would look totally foreign to the Democrat coalition of the FDR and JFK era -- what happened to the working class and labor unions, the backbone of the New Deal coalition?
Third parties do occasionally achieve national success, but they are short-lived reactions by defectors from one of the two parties, intended to punish the other members of the coalition who have betrayed the defecting group. They realize they will not win the general election as a break-off faction of one of the two parties -- the point is to punish past wrongdoing within the party, and serve as a credible threat against any future betrayal within the party.
Importantly, they are swift responses against the incumbent party -- not delayed grudges.
Nader 2000 was mostly a reaction against the Democrats selling out during the Clinton era of elitist and globalist New Democrats. Perot '92 and '96 was a reaction against both the elitist / globalist policies of the Bush Sr. party, as well as the incipient New Democrats. Anderson '80 was a reaction by former Carter voters who wanted more of a moderate who didn't mention he was a born-again Christian. Wallace '68 and Thurmond '48 were both reactions by Deep Southerners who didn't like where the Democrats were headed with desegregation, during and after WWII, when the Democrat administration desegregated the Army. Progressive Party runs by Roosevelt '12 and La Follette '24 were both reactions against the Republicans for becoming too conservative.
Really the only third-party campaign that consistently broke into single digits with the national popular vote was the Socialist Party in the early 20th C., a social-democratic party that was not a break-away from either the Democrats or Republicans. But with both major parties including the working class in their coalitions -- first the progressive Republicans, and later the New Deal Democrats -- there wasn't enough reason to go outside into a third party based mostly on labor rights, with no broader coalition to build. No broad coalition means no chance at the national level.
So what does this bode for the current season? If anyone is going to break off from the Republicans, it's the Cultural Right / Tea Party. Enough of them seem to be on board the Trump train, though (maybe 50%), that they aren't cohesive enough to make a run of their own. If they did, it would be in the heart of the Cuck Belt, the Plains, a la Ben Sasse continuing to pipe up about a "consistent conservative" candidate.
However, the Republicans haven't held the Presidency for eight years, so it's a bit late to launch a retaliation to punish a betrayal from the '00s. That was the Tea Party Congressional landslide of 2010, and that's already run out of gas, not to mention getting eclipsed by the Trump phenomenon at the national level in 2016.
If anything, it would be progressives bolting the Democrats to punish getting sold out by Obama and Crooked Hillary. Unlike the Tea Partiers, the "Bernie or bust" people still have fresh wounds and a bad taste in their mouths. Whether they draft Bernie to run as an independent, write him in, or flock to the Green Party, remains to be seen. A good chunk of blue-collar Sanders supporters will come around to Trump, another good chunk will stay home, and only a handful will turn out for Clinton.
The progressives, though, are a separate faction within the Bernie coalition. They won't vote for Trump, and they seem too energized to wind up staying home in November, after the superdelegates deliver the nomination to Crooked Hillary on a silver platter. It could be a Perot-sized rift on the Democrats' side, which would help Trump pick up divided blue states that would otherwise be an uphill battle (Colorado, Washington), in the same way Bill Clinton picked up red states that were divided by Perot (Georgia in '92, Arizona in '96).
Everything is lining up for a wipe-out victory for the Trump movement. The only likely third-party rift is on the Democrats' side, and the Republicans are quickly re-aligning to shed dead weight and appeal to a much wider base, who are growing the party in record numbers. The Democrat Establishment is only bent on worsening their own problems -- antagonizing the Bernie crowd (e.g., the Nevada Convention this weekend), defending the superdelegate process, and courting neocons and Wall Street mega-donors from the Establishment Republicans who are leaving the inchoate Party of Trump.
It's gonna be epic.