Extensive Study of Third Party Candidates for Office over Last Decade Published by Follow the Money

From the Executive Summary of the study at the National Institute on Money in State Politics, via Poli-Tea:

Voters in the 2010 elections can again expect to see few third-party candidates who seek to provide alternatives to politics as usual. Those few candidates face long odds. Of the 6,181 third-party candidates registered in the last nine years, only 2 percent won their races. Moreover, the traditional advantages of money and incumbency don't confer success onto third-party candidates to the same extent as they do mainstream party candidates.

In the last nine years, only 2 percent of third party candidates won their race

• Third-party candidates financed their own campaigns more often, and lost more often, compared to self-financiers in the mainstream parties.
• Candidates identifying themselves as Independent and Progressive Party candidates were most successful; candidates from the Libertarian and the Green parties were among the least.
• Third-party candidates fared best in Vermont: 34 candidates out of 236 won seats, or 14 percent. Kentucky saw the fewest third-party candidates: only 15 filed for office during the study period 2000-2009. In California, 392 third-party candidates sought office—none won.

One might dispute the statement that there are "few third party candidates." Poli-Tea's list of third party and independent candidates is not nearly complete, at it logs hundreds of candidates for governor, US House and Senate. The Libertarian Party alone is itself running hundreds of candidates this election season.