The political talking heads are working themselves into a minor frenzy over the possibility of a Hillary Clinton vs. John McCain presidential matchup in 2008. Actually, it could get even more interesting with the real possibility that a major third-party candidate will enter the race. In fact in every election since 1992 (with the exception of 2004), third-party candidates have played a major role in shaping the campaign and determining the outcome. With significant majorities of Americans registering disapproval of the job done by George W. Bush and both parties in Congress, it is not surprising that more than half of the electorate sees the need for a third party. Nor, given the difficulties politicians have had in dealing with crucial national issues, is it surprising that former Federal Reserve Chair Alan Greenspan has already jumped aboard the 2008 third-party express.

Could a serious third-party challenge develop again this year? Much discussion of likely third-party candidates has focused on centrist candidates. This makes sense given what many see as the problem: both parties dominated by extremists, while the electorate remains -- in Greenspan's words -- "a vast untended center from which a well-financed independent presidential candidate is likely to emerge in 2008." Greenspan left the identity of this "well-financed independent" to the imagination, although New York City Mayor Michael Bloomberg has become a hot topic of speculation.

But centrist third parties in America have rarely been successful in winning even 5 percent of the presidential vote. Rather, in virtually every case of a successful third party, that party identified issues ignored by the major parties and took positions that were relatively extreme -- George Wallace on race, the Populists on the silver standard, and the Republicans on slavery. It was not by "splitting the difference” between the two major parties, but by "showing the difference" that these campaigns succeeded.

The plausibility of a centrist run is reinforced for some by the example of the most recently successful third-party candidate -- Ross Perot. To many observers Perot's campaign was one that succeeded because of his centrist appeal and his wealth. Perot was different from previous successful third-party candidates because he had "more money than God" (although less than Michael Bloomberg). But Perot led the presidential race at a time when he had spent very little -- not only spending less than a third of what either Clinton or Bush Sr. had spent, but less than also-rans Paul Tsongas and Pat Buchanan. Money is far from irrelevant, but neither is it a panacea. It was clearly not Perot's campaign spending that propelled him to the lead in the spring of 1992.

Neither was it his moderation. In our new book, Three's a Crowd: The Dynamic of Third Parties, Ross Perot and Republican Resurgence, Walt Stone and I find that, even though Perot and his supporters were moderate on traditional partisan issues, these were not the issues that really mattered to them. As his campaign manager, Tom Luce, put it: "In the entire time I worked in ... the Perot campaign, I never had a single volunteer ask me if Perot was conservative, liberal or moderate. As far as they were concerned, those were meaningless terms." What Perot offered was a new agenda. Perot and his supporters were relatively moderate on issues of abortion, affirmative action and gun control -- but these issues were not what drove the fanatical devotion to getting Perot on the ballot in all 50 states as he demanded as his price for entry. What drove Perot's supporters were the issues that the major parties ignored, like the budget deficit and political reform, as well as issues that both parties had similar positions on, such as NAFTA. These were the issues that mattered to Perot and his supporters, and on every one of them Perot was more extreme than either the Democrats or the Republicans.

So, what is the lesson of the Perot campaign for 2008? Third-party candidates succeed not because of their centrism, but because of the rest of their issue agenda, which is not centrist. To be successful, a candidate must be armed with an agenda which encompasses issues that do not fit neatly into the left-right dimension which currently divides the major parties. Thus John McCain would be a formidable third- party candidate, not because he is a centrist, but because he embodies a broad reform agenda that neither party can fully embrace. Lou Dobbs would be a formidable candidate because his stance on the immigration issue imparts a clarity absent from either major party. As for Michael Bloomberg, or anyone else Alan Greenspan had in mind to achieve the same level of success as Perot, he needs to be clear about what sets him apart from the status quo -- on reform, immigration or, as author and columnist Thomas Friedman advocates, on the environment. Once that is done, the three-party contest can begin.