The problem with Ralph Nader is not that he is wrong on the issues (he rarely is), nor that he does not speak truth to power. The problem with Ralph Nader is that we only see him on TV six months before a presidential election. That's it. After that, he all but disappears into the political backdrop, getting even less attention from the media than he does during the election cycle.
Yes, those who frequent certain websites and who spend a considerable amount of time scouring the Internet for real news can always find an insightful article by Nader, like this one, in which he asks people to support H.R. 676, and (indirectly) oppose H.R. 3200, which is the pro-corporate interest bill that is currently in pole position to be shaped into law. But it is a pity that more people do not recognize that until we keep voting for a candidate from the two-party oligopoly that holds our hopes hostage, we will hardly get anything better than what we've gotten for ourselves over the course of the past 233 years, particularly in our recent history.
Don't get me wrong: There are plenty of smart, principled, dedicated, and (relatively) honest congressmen and women around. But for them life in Congress is more frustrating than rewarding.
The American political system we have now is not built to promote change and progress, it is built to preserve the status quo. As you probably know, over the last six election cycle the re-election rate for incumbents in the House has never dropped below 94%; and, since 1964, it has never dropped below 85%. Re-election rates for incumbents in the Senate are a little lower, but never lower than 79% in the last 20 years, but then senators serve for 6 years.
The current system is not equipped to represent and give a voice to all the diverse opinions and positions that exist in the American society, and this is the fault of the American people, for never demanding and mobilizing for anything different. On the contrary, with the exception of the last couple of elections, in which a society largely polarized for or against the Bush administration voted in somewhat larger numbers than usual, the political system we have now is designed to engender apathy in voters.
Right now, the current two-party system favors Democrats, simply because there are not enough centrist and moderates around who support the extreme agenda and positions of the Republican party. In the short term, a move away from a two-party system is likely to benefit the weaker of the two dominating parties (currently the Republican Party). In the long term, though, it is likely that a system featuring a higher number of choices would revitalize the political scene, simply because Republicans and Democrats would have to work much harder (or work at all) to regain the confidence and the commitment of voters who have abandoned them for better, more honest choices.
The fact that Ralph Nader and his supporters are incapable of producing enough steam to keep their ideas at the forefront of American politics is indeed a direct consequence of a rigged system, but it is also a failure of their imagination to devise and achieve a consistent presence in American discourse. It takes time, money, and effort to achieve "party" status, and this is the only thing that Republican and Democrats have been any good at.
So, I guess, in the end the problem with Ralph Nader, and with all the other characters that from time to time pop up in American politics, is that they have not been able to remind the American people forcefully enough that the definition of insanity (or, perhaps, stupidity) is doing the same thing over and over again and expecting different results.