Prior to FDR, no one in the mainstream political system even thought about representing lower income, minority or worker interests, and FDR certainly didn't--at first. There were third parties, like Norman Thomas's Socialists, who talked about it; they never successfully did represent workers, however. Later, Roosevelt did. I heard NormanThomas boast (in the 50's) that virtually all of his 1932 platform had been implemented by FDR, Truman and Eisenhower. In the last 25 years most of it has been dismantled.
This history illustrates one of the positive aspects of third parties: they can lead the agenda, play a visionary role and generate ideas; the only third party that was successful in carrying out its agenda, however, was the Republican Party of Lincoln, but then the second party (the Whigs) had virtually disappeared. The GOP became the dominant party until 1932. Like the Socialists of the 30's, most third parties--except in fusion politics--generate ideas, popularize them, but can get no further than that, because of the structural obstacles built against third parties.
But there are negative aspects to third parties as well: they can throw elections against their and the majority's interests, because of these same structural obstacles. Until, or unless proportional voting, instant run-off, or multi-candidate districts is adopted, and until the obstacles to getting on the ballot are removed, those negative effects can be disastrous--as they probably were in 2000.
As a consequence, not only has the party of big business and inherited wealth depended upon large, wealthy donors, but the party that claims to be for "the little guy" has had to seek large donations from the wealthy as well, in order to be competitive: how else do you buy air-time and effective organization? Both cost money in this marketized nation. The result: the Democratic Party has been supported financially by the same donor class that supports Republicans, and Democratic candidates cannot appeal to worker and minority communities without threatening their campaign finance supporters. "Class war" becomes prohibitively expensive for the lower income side of the equation, but not for the party of the wealthy. That's why there has been little in the way of liberal economic initiatives in this narrowly Democratic Congress. That's also why Obama's movement, his online fundraising from 2 million people, could be significant.
The problem for third parties, especially on the left, really boils down to this: they cannot gain enough funding to mount credible campaigns. That's why their campaign positions can be pure, so perfect to the discerning; that's also why they only rarely win elections: few people know they exist, or, when they do, they don't look credible. Conservative third parties can raise more money from a few wealthy donors, but the narrowness of their base tends to make them less credible as well. That's why third parties rarely win even local elections; even George Wallace and Ross Perot came in distant thirds, although their vote tallies were higher than any other national third parties. The election system was tailored for two parties after all: winner take all, plurality wins, no run-offs (with a few exceptions), expensive, privately funded campaigning, including advertising at market rates, and difficult hurdles set up for any party that wants to get on the ballot: third parties spend much of their campaign funds simply to qualify; the two party duopoly spends its considerable funds on the actual campaign.
So, when is a vote for a third party candidate justified?
Many progressives are now saying that Obama is "corporate," that he's made too many concessions to monied interests: defense interests, coal interests, financial interests, or more broadly: corporate interests. Some of these people say they'll still vote for him as "the lesser of two evils," but others are saying they'll vote for a third party candidate who more closely represents their interests. I'm arguing here that under many circumstances the second alternative is not rational. It isn't to vote for Bob Barr instead of McCain, either.
In a world of proportional representation, or instant runoff voting, or multi-seat constituencies, voting your conscience makes sense: you can vote for your guy and know that if he doesn't make it, the next best alternative, ranked as your second choice, would not be hurt by your vote.
But, consider election 2000. Nader's own polling showed that if he had not been on the ballot in Florida, the overwhelming majority of his votes would have gone to Gore; Gore's margin would have been large enough that a recount would not have been necessary. Instead, GW was selected--the margin was small enough that the Supreme Court could effect its judicial coup. The effect for Nader voters was that their vote enabled the candidate to be elected who was most antithetical to their politics. The consequences we all know: Afghanistan, Iraq, torture, tax cuts for the rich, anti-environmental policies, spying, a radical right Supreme Court, and on, and on. Most of these things would not have happened with Gore: surely these were not policies Nader voters were voting for.
If voters voted rationally (some do), they would vote their conscience only when the likely result was not going to be most antithetical to their interests. Here in New York it is virtually certain that Obama will carry the state. Therefore, if you felt that he had compromised too much and that he should at least know that there are more liberal voters out there you could vote for the Green candidate, Cynthia McKinney, or for Nader; you could be sure your vote wasn't helping McCain win the Presidency, but you were expressing your protest, or your convictions.
But here is another example of rational voting. I have a Green friend who acknowledges that Obama is markedly better than the "Nazi" McCain, even though Obama is "corporate," in his estimation. For that reason, he plans to volunteer to work on the Obama campaign in Pennsylvania, which will be a "battleground" state, one that Kerry carried by a whisker, but that McCain might have a chance of winning, in part because of the difficult racial relations there. But, while my friend will work for Obama in PA, (and campaigned there for Kerry four years ago) he'll vote for McKinney in NY. It makes perfect sense from his point of view: he'll be working to help the lesser of evils to get elected, but he'll register his protest against "corporate" politics by his vote in NY.
I can't do that. I'll work for Obama here (NY) and also probably in PA. I voted once for another third party candidate (Barry Commoner) in Florida and against both Carter and Reagan. Reagan won Florida, and I felt personally responsible for years afterwards.