> There are two successful scenarios for a third party (if the US keeps its single-district first-past-the-post electoral system). The first would be a gradual frittering away of Democratic support in favor of the third party (think Liberal Party and Labour Party in Britain).
I don't think this is a relevant comparison. In 1912, the U.S. Socialist Party won 6.0% of the Congressional vote; in 1910, the British Labour Party won 6.4% of the parliamentary vote. But the U.S. party obtained zero seats, while the British party obtained 42 seats. I don't know if this was because of (possibly tacit) cooperation b/w British Labour and Liberals, or because of differences in the geographic concentration of the left. But either way the analogy didn't work then.
> The second would be a crisis that produces a sharp electoral realignment in a short period of time (think
> Whigs and Republicans in the U.S.)
The problem with this comparison is that this sort of realignment has *already* happened. The realignment of the 1850's was about a shift in the ideological basis of party competition - the parties stopped being two coalitions of pro- and anti-slavery forces that differed on economic issues and became one pro-slavery party and one antislavery party. A clear choice. Likewise, before the 1960's the two parties were ideologically heterogeneous coalitions. But then a gradual realignment happened that made them differ strictly according to left-right ideology: the GOP is clearly the party of the right and the Dems are clearly the party to the left of the GOP.
So what would a new realignment be about? It would just be a general shift to the left, not a realignment. In other words, we would end up with a DP further to the left, but still the DP.
> The odds of either scenario working are very low, and those who support a third party are either dismissed as irrelevant or, in exception circumstances, blamed for Republican victories (Nader 2000). Most leftists, therefore, stay out of electoral politics and focus on the Sisyphean task of putting on pressure from the outside (and their liberal allies desert them in this task when Democrats are in power). Outside pressure sometimes works and sometimes doesn't, but the crucial variable isn't how hard the leftists work at it (imo it's how divided capitalists are). If anyone knows a way out of this conundrum, please let me know.
I think you just have to expect that neither electoral nor extra-electoral pressure will accomplish much as long as there are so few leftists. Pressure would be effective with more leftists. The question is how to manufacture them.