Roughly half of all positions are democrat held, the other half republican held.

Its more 55/45, as Republicans dominate the rural sectors with a plurality of smaller seats while Dems dominate the large-pop singular seats. Even then, the real balance of power is in the financing of races, with local business magnets and special interest groups dictating the nominees of both major parties. Down in Houston, for instance, the candidates that consistently win the mayorship have to first win the endorsement of either the police or fire departments (ideally both), as these organizations command popular prestige, enormous campaign war chests, and a large body of active canvasers who will work on your campaign's behalf. Similarly, everyone kowtows to the oil and gas industries.

Introduce an independent candidate for mayor, and that candidate will still need to suck up to O&G, fire, and police in order to win the race. And, once in office, they'll be constrained just like either of the two major party's preferred candidates would be.

Go up to the Rust Belt and you're bowing to the interests of the automotive industry. Take a stab at politics on the West Coast and you're going to need to cater to Silicon Valley. Everyone running for office on the Atlantic Seaboard is keenly aware of the clout enjoyed by the investment banks, the real estate magnets, and the DC bureaucracies. Add a modern-day Ross Perot to your list of candidates and you're still juggling these interest groups in order to win.

Take the green party for instance. They definitely don’t align with the democrats, but they can at least agree on some things, where as them agreeing with republicans is far, far more rare. So it is in the interest of green voters that green politicians get voted in most, followed by democrat politicians, then republican.

But when they split the vote due to the spoiler effect, it ends up meaning the worst of the worst options gets voted in, a republican. And that should matter to the 3rd party losers.

Except you're assuming people are choosing to vote D, R, or G and then ranking their preference. In truth, you've got a substantive pool of voters who simply do not turn out when they don't like who is on the ballot. Turnout in the US rarely breaks 60% of the eligible base. But when it does, you can see establishment candidates falter behind insurgents.

At that point, the Ds and Rs will court you for your membership in their party. And because they have far more to offer than a Green Party leadership, more viable candidates tend to be attracted to the Big Two parties. Greens (and Libertarians and other niche parties) are stuck with candidates who can't get onto the D or R ticket via a primary or appointment.

If you have a candidate that is genuinely popular and generates a ton of organic enthusiasm - a guy like Trump, who bounced from Reform Party to Dem Party and then on to the GOP unsuccessfully for decades, before catching fire among anti-Obama anti-immigrant conservatives - then that candidate is going to dramatically increase voter participation and win regardless of which party they run under. Similarly, Obama was able to undercut Hillary in the '08 primary by dramatically boosting turnout, particularly in states where Hillary failed to campaign aggressively. So Tennessee and South Carolina and Georgia went to Obama in landslides, undercutting Hillary's thinner margins in California and New York and Florida.

Obama could theoretically have run as an Independent candidate for Senator of Illinois and won (in large part thanks to the Republican incumbent flaming out in a sex scandal), then challenged the Top 2 for the Presidency. But why do that when you can run inside the party apparatus and fall back to a cabinet position or VP slot if you lose?

The spoiler effect doesn't come into play, because the people who have the most viable campaigns get absorbed by the bigger parties. That's why Sanders and Angus King caucus with the Democrats and even run as Democrats on the Presidential ballot.

As another case-in-point, consider Dan Osborn, an Indie currently being courted by the Democratic Party entirely because he polls so well against sitting Senator Deb Fisher.