In the comments to the post on moving American politics to the left, a number of people have talked about the value of 3rd parties in turning the main 2 parties to your position. I am skeptical and I think that a cursory look at the major 3rd parties in American history suggests that the argument for the efficacy of 3rd parties is only tenable in its most shallow form.
1. The Republican Party–The Republican Party really isn’t a 3rd party but people call it such. What was happening is that the Whig Party had collapsed over the issue of slavery. Something had to replace it because it was no longer a tenable entity. The Know-Nothings nearly won the day and probably would have had the South not overplayed its hand with the Kansas-Nebraska Act. But there is little to no relevancy here for thinking about 3rd parties today.
2. The Liberal Republicans–The classic top-down 3rd party with zero staying power, the Liberal Republicans of 1872 consisted almost wholly of Republican elites who didn’t like that Grant was trying to enforce Reconstruction. Disappeared by 1873. Absolutely no relevance to the present except as an example of how limited top-down 3rd parties are.
3. Populism–This is the most complicated. People cite the Populists as moving the Democrats to the left. Maybe. Certainly Bryan was to the left of Grover Cleveland. But it’s also important to note that many of the actual reforms desired by the Populists were accomplished by the Progressives, who were by and large Republicans. When Wilson became the next Democratic president, he was probably the least reform-minded of the 3 candidates in 1912. If you look at his record, he got a bunch of legislation passed in 1913 and then did as little as possible until he realized that the nation was demanding more reforms and he needed to get something passed in 1916 in order to get re-elected. Of course, 1916 and 1896 are a long time apart, but it’s not like the Populists somehow made the Democratic Party a standard-bearer of the left. Bryan won the nomination twice, but in 1904 it went to Alton Parker, who was more in the mold of Cleveland than Bryan.
The Populists were also only sort of a 3rd party. As several historians have noted, the Populists made hay where, in effect, states were controlled by a single party, whether the Democrats in the South or the Republicans on the Plains. They were a response to the complete inability of people to influence the shockingly corrupt state legislators of the Gilded Age. In states that had legitimate two-party competition, the Populists made very little headway.
4. The Bull Moose Party. Another top-down entity that disappeared as soon as Roosevelt threw the election to Wilson. Arguably the apotheosis of the Progressive movement, but utterly without cohesion and really made little difference in American history. Gets a lot of play because of the TR self-promotion machine, but of limited real importance.
5. The Dixiecrats (and other segregationist third parties of the civil rights era)–These were a response to changing party and regional dynamics and were part of a fairly natural realignment. Not sure what relevance there is here to today.
6. Henry Wallace’s Progressive Party–A complete failure. Did not turn the Democratic Party to the left. Marginalized and soon forgotten.
7. The Green Party–If Nader hadn’t thrown the election to Bush, it would be a total irrelevancy. It certainly did not move the Democratic Party to the left. If anything, it increased the marginalization of the party’s left-wing by showing the lack of commitment to party structure among liberals.
So I don’t see any useful examples from the past that suggest creating a 3rd party is going to do anything other than throw elections to Republicans. The only reasonable possibility is along the lines of something Historiann suggests, that the Democratic Party no longer remains a functioning entity and that a new party rises from its ashes like the Republicans in 1854. There are 2 problems with this. First, I don’t know what the issue is that would make the Democrats crumble as a structure. There is nothing like slavery out there; if anything, such a scenario would see Republicans break over abortion. Except, and this gets to the 2nd problem, right-wing Republicans showed how to take over a party structure rather than blow up the party. That’s how you get things done. These calls for a 3rd party always assume that they will be of The People and will somehow remain immune to big-money politics. And I don’t see any evidence for that.
The only solution is to take over the Democratic Party from the inside.
Update–It’s been noted that I forgot the Socialist Party, which made its great stand in 1912. Not sure this changes anything though. Like the Populists, the Socialists were part of a huge upheaval of demands for social change in the late 19th and early 20th centuries. While Eugene Debs was a powerful voice for the poor and while the Socialists had success in winning local elections in Midwestern cities such as Milwaukee, I don’t think you can make an argument that they really moved either major party to the left. In 1912, all 4 legitimate candidates were running on platforms that called for some level of social change.