I just don't see a 3rd party EVER coming to any kind of significant influence over election outcome. The only guy who came close was Ross Perot of the so-called Reform Party in the 1992 and 1996 elections and that did nothing but put Slick Willie in the WH.
One could argue that Perot's startling performance gave rise to the Tea Party (he won more than 5% of the vote), but I note that there really haven't been any candidates that split the Dem party nearly as much as Independents have split the Repubs.
In every instance I can think of save 1968's George Wallace who failed to keep Nixon out of the WH, the rise of a 3rd party candidate put a Dem in the WH.
Besides, the American voter just doesn't veer very far from the 2-party system, as corrupt and evil as it is.
I'd love to be able to vote the Constitution Party and be reasonably assured that my vote had meaning, but in today's political system, me voting for a Constitution Party candidate is a vote for a Dem, and that is unacceptable.