As Ralph Nader considers whether to run again in 2004, and as the Green Party fields candidates on ballots throughout the United States, these dedicated progressive activists should consider one clear fact: They compromise the very causes they seek to advance, and in direct proportion to their success.

One can entirely empathize with the impulse to vote Green or for another third party. But in the U.S. two-party system, that impulse carries with it a cruel irony: Any third party splits the vote on its side of the right/left divide. The more successful the third party gets, the more it ensures the election of the very people most hostile to its cause. This is an undeniable reality -- until, of course, the third party becomes the majority party, a result no rational observer contends will occur in this country for decades at best.

There could be no more eloquent testimony to this effect than our present political circumstances. In the 2000 presidential campaign, based on exit polls cited by Nader himself, 38 percent of his voters would have voted for Gore, and 25 percent for Bush. Regardless of other factors (Gore campaign errors, GOP misdeeds, the Supreme Court, etc.), Gore's net loss of 13 percent of Nader's 97,000 votes in the state of Florida put George Bush in the White House. The result? Looking back at the past few years, progressives can be nothing short of horrified at the Bush presidency's impact on their most cherished issues.

This hard reality for third-party politics in America does not apply elsewhere in the world. In a parliamentary form of government, dozens of parties can each have a few seats in parliament, which then elects a prime minister. In that political environment, third or even 15th parties make all the sense in the world, and they can wield great power, as European Greens and Israeli religious parties have amply demonstrated. But that rationale is entirely absent in the U.S. context.

Citing the most fundamental of American values, many Green and Nader supporters will at this point in the argument charge that discouraging third parties is a direct assault on democracy, a restriction of freedom of choice. Nader himself asserts that calling him a "spoiler" is just such an affront, and that he has every right to run. This response entirely misses the point: No rational person says third parties don't have a right to run, but rather that by running effectively, they doom the issues they care about.

The closest thing to a logical argument for Nader or Green candidacies is that they will so vilify Bush and the GOP that voters will turn elsewhere. But for that to contribute to Bush's defeat, one must simultaneously accept the contradictory conclusion that a voter thus inspired by Nader and Green arguments will then vote Democratic, and not for the person or party they found so persuasive. Effective Green candidates will win votes, but to the extent they are successful, they will cost Democrats vastly more elective offices than they gain for themselves.

Happily, there is a powerful and rational alternative to the purist third-party approach.

In our two-party system, the best scenario for Green-leaning independents is to forge a vibrant, grass-roots "GreenDem" wing of the Democratic Party, and fight like crazy to mobilize nonvoting Americans to whom they will have special appeal. Then something great will happen: Democrats will be elected nationwide, and progressive activists will be a decisive interest group with genuine leverage over politicians who are actually in power. The alternative is the sad spectacle of a group that is ideologically pure, but with no influence whatsoever, forced to simply stand in horror as the Bush administration and Congress smugly, gleefully destroy each item on the progressive American political agenda.

Imperfect as it is, in the two-party system, the perfect is the enemy of the good. And the good is crucially better than the catastrophic.

John Pearce is the creator of RalphDontRun.net, a Web site encouraging progressives to ask Ralph Nader not to run in 2004. He is founder and former CEO of MediaMap.