> A third party doesn't actually don't need to win. If a candidate does not receive a majority in the electoral college I believe the house / congress decides[1].

(1) a third party is unlikely, in failing to win, to get even a single electoral vote, much less be instrumental in denying the leading major-party candidate a majority.

(2) If no candidate gets an electoral majority for the Presidency, the House elects a President on a one-vote per state basis (and if they don't do it by a specified time, the Vice President becomes President); if no candidate gets an electoral majority for the Vice Presidency, the Senate elects a Vice President.

> This would also pretty much destroy both parties - they will have to compromise and come closer to the middle.

Why would it destroy both major parties -- particularly the one that still wins the Presidency in that scenario?

> Basically, if a third party can get even 15% of the vote it would open the door for another candidate.

No, it wouldn't. We know that, because its happened, not that many elections ago (H. Ross Perot, 18.9% in 1992.)

So you're ignoring the self-implosion of the Reform Party (after it's takeover by Pat Buchanan) as a factor to their not being re-elected?

You're also ignoring all the changes made to election rules since Perot (all made to prevent such a strong showing again)?

Fine, swap an impossible reality for a slightly-less-impossible reality. I feel just as confident saying "a third-party will not keep a candidate from winning the electoral college" as I do saying "a third-party candidate will not win the election." Third-party candidates do not siphon votes equally from the two major parties, they typically just make it easier for one to beat the other. I think it more likely that a third-party candidate could get as much as 30% of the popular vote without winning a single electoral vote.