Lots of speculative buzz out there about possible third party candidates and what they might do to President Obama’s hopes for re-election. Theo Anderson, for example, has a post up at In These Times, “Why Gary Johnson Should Terrify the Democrats,” arguing that,

The conventional wisdom is that a challenge by a strong Libertarian candidate would hurt the Republican more than the Democrat. But that seems unlikely. Democrats are usually critiqued from the right and pulled toward the center. The pressure coming from the GOP is always in the direction of more defense spending, a more hawkish foreign policy and fewer civil liberties. But what if Democrats are seriously challenged from the left on social and foreign policy-by a self-styled conservative?
The danger for Democrats isn’t that Johnson will win a significant percentage of the Left’s vote. The danger is that he’ll peel away a sizable share of the much-prized independent voters, who tend to be fiscal conservatives and social liberals, and who might feel, understandably, that Obama hasn’t played it straight with them. He backed away from his early-career support for gay marriage rights, for example, and endorsed civil unions when he became president. His position is now reported to be “evolving.” Does anyone know where it has evolved to, or when he might come to a definite conclusion? Or where he’s at on immigration reform? Or on the drug war?
There’s no uncertainty about where Gary Johnson stands on those issues. On every one of them, his position is both clear and deeply offensive to the GOP base, which is why he never had a chance of winning the Party’s nomination and has very little chance of winning the presidency. But it’s exactly why he appeals to independents.

Anderson’s rationale seems a little tortured, especially in stark contrast to Johnson’s limited charisma. Mr. Excitement he’s not, which is one reason he tanked in December, while quasi-libertarian Ron Paul is still a nettlesome factor in the GOP field.
Moreover, Anderson makes the classic mistake of treating Independents as a real-world third force, which they are not, as Alan I. Abramowitz has made clear with hard-headed data analysis on many occasions. From Abramowitz’s most recent post at Sabato’s Crystal Ball:

…There’s an organization that hopes to provide Americans with a centrist alternative to the two major party candidates in 2012. It’s called Americans Elect and it has already raised over $20 million…The absence of a high profile candidate is far from the only major obstacle that Americans Elect faces. Attracting media coverage, raising the hundreds of millions of dollars needed to wage a national campaign and securing a place on the ballot in all 50 states are perennial problems faced by third party candidates.
Of course there will be third party candidates on the ballot in 2012, just as there are in every presidential election. But it is unlikely that any of these candidates will approach the 19% of the vote that Ross Perot received in 1992, or even the 8% that he received in 1996.
Third party candidates have not fared well in recent presidential elections: The total vote won by third party candidates has fallen from 20% in 1992 to 10% in 1996, 4% in 2000, 1% in 2004 and 2% in 2008.
There’s an important reason why third party candidates have fared poorly in recent presidential elections and why third party candidates are likely to fare poorly again in 2012: partisan polarization. The vast majority of American voters today, in fact well over 90%, identify with or lean toward one of the two major parties. And the vast majority of those identifiers and leaners strongly prefer their own party’s candidates and policies to those of the opposing party.
…Over time, the parties have been moving apart. But both Democrats and Republicans are now closer to their own party and farther from the opposition party than at any time in the past four decades. Democrats on average place the Democratic Party exactly where they place themselves while they place the Republican Party very far to the right of where they place themselves. And Republicans on average place the Republican Party exactly where they place themselves while they place the Democratic Party very far to the left of where they place themselves. As a result, very few supporters of either party are likely to be tempted to vote for a centrist third party.

As for “Independents,” Abramowitz clarifies the ‘threat’:

There is one group of voters that might be tempted to vote for a centrist third party: pure independents. These voters, on average, place themselves right in the middle of the two major parties and rather far from either one. But pure independents typically make up less than 10% of the electorate, and they tend to be less interested in politics and less attentive to political campaigns than voters who identify with a political party. There are simply not enough of them and they are too hard to mobilize to have a major impact on the outcome of a presidential election.

None of this is to say that is it impossible for a third party candidate to do significant damage to the Democratic nominee, as many believe Ralph Nader did in 2000. But it is unlikely, especially with the existing possible third party candidates, none of whom appear to have the chops to bust the two-party paradigm.