A fringe candidate can steal the occasional House or Senate seat. I really don't think the Tea Party are "fringe." They are well funded, have a whole news channel supporting them, and I don't think it is a coincidence that they really took off right around Citizens United. They are a sort of amalgamation of bits of the far right wing Christian section of the Republican Party. These bits have always been there. Libertarian, Christian, anti-government, social conservative...these are pillars of the Right wing of the Republican party. They have shifted some Congressional elections and become a nifty foil for some batty candidates to get way more recognition than they deserve (Palin, Cruz, Allen West, Bachmann) but as we saw in all the Presidential elections since their inception, the GOP primaries have eventually settled on the "safe" popular candidates that can pull Independents and Moderates (see e.g. Romney, McCain). These safe candidates ride the crazy train while they have to, knowing that eventually their well entrenched political machines and presence in the "real" GOP establishment will secure them the win.
A better demonstration of the "right wing" fringe is Ron Paul. He is the Republican party's Nader. No chance of ever winning a general, but always runs. Always pulls some supporters and has popularity, but even though the GOP knows he is going to pull some support every time, he has not had a profound effect on party politics. Rand Paul, arguably his successor, is in fact moving more to the "mainstream" than his Dad ever did precisely because he knows he cannot pull the party in the other direction in a Presidential primary.
To get an idea of how utterly impossible it is in the modern American political system for a third party candidate to have any effect whatsoever, look at the 1992 General. Ross Perot took 19% of the popular vote, the highest % of voters for a 3rd party candidate since Teddy Roosevelt. He siphoned off voters pretty equally from Bush and Clinton, thus representing a truer "third" party candidate than most. Guess how many States he actually carried in the electoral college? Zero. How much effect did he have in the 1996 election? Less, actually. He got only 8% that time. NAFTA was ratified even though he was against it during his candidacy, Reform party candidates Buchanan and Jesse Ventura faded into obscurity, and Perot himself later went on to endorse mainstream GOP candidates W. Bush and Romney.
It is a sad fact of American politics that in the General election, if you live in a state that is contested, you have to vote for the candidate who you like the most who also actually has a chance to win.