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.
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.
The US election system requires huge amounts of money, in part because of the pro-market positions of both parties, but especially of Republicans, and their success in seating courts which rule in favor of the power of money. Buckley vs Valeo (1976) sealed this process, by ruling that money was equivalent to political speech, and therefore protected under the First Amendment. Ever since, campaign finance reform has been rendered ineffective. Money talks.
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.
So, voters are faced with a dilemma. Often, the candidate/party that most closely represents their interests or view of the world is a third party that cannot win, yet their platform and candidate statements make the most sense, and are the clearest articulation of their own views. On the other hand, the major parties are compromised by having to compete against each other with large amounts of money (probably over a billion dollars in this election cycle). Without the money they would be invisible, but the money comes with tacit conditions, such as: remember Wall Street interests when attempting to deal with the credit crunch. Popular opinion doesn't count for as much on issues like that; money opinion does.
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.