The question I get more than any other about American politics is: The Democratic Party and the Republican Party both suck. Don't we need a third party to fix this?

Well, to paraphrase Bill Clinton, it depends what the meaning of "this" is.

If you think the problem with American politics is that there are ideas that are popular among voters but suppressed by the two major parties, then a third party could potentially help a lot.

But if you think the problem with American politics is that Congress is gridlocked, the president seems powerless to do anything about it, and Americans are increasingly frustrated, then a third party might well make things worse.

The case for a third party

Political scientist Ronald Rapaport wrote the book on third parties. Literally. It’s called Three’s a Crowd, because of course it is. And the key thing he found about third parties is that "they need some sort of unique agenda. There has to be a reason why you’re going to support a third party."

Third parties are a political weapon: they force the system to confront issues it might otherwise prefer to ignore. Take Ross Perot, the most successful leader of a third party in recent American history. "People like to think of Perot as being centrist. But he was not," says Rapoport. "He was extreme on the issues he cared about. And with Perot, it was economic nationalism and balancing the budget."

It’s worth stopping on that point a moment. In Washington, the yearning for a third party is often by elites — and for elites. It’s for the third party of Unity08, or No Labels, or Mike Bloomberg, or Simpson-Bowles. It’s a third party of technocrats: fiscally moderate, socially permissive. A third party of sober moderates. A third party of things people in Washington already care about.

That third party won’t work. The space for a third political party — if it exists — isn’t in Washington’s zone of elite agreement. It’s in the zones of popular agreement that elites have little patience for. America’s unaffiliated voters aren’t moderates. They are, by Washington’s standards, extremists — they’re just extreme in a way that blithely crosses left and right lines, then doubles back on itself again. They support single-payer health care and tax cuts. Or they’re against gay marriage but for a living wage. Or they're for open borders and cuts to social spending. Or they want a smaller military and sharp restrictions on abortions.

Perot’s enthusiasts were a good example, Rapoport says. "His supporters, on issues like choice, were very pro-choice. On affirmative action, they were very against affirmative action."

Third parties like Perot’s can force issues to the fore. But, typically, they get co-opted. Bill Clinton was much more intent on reducing the deficit because Perot showed the issue’s power. Newt Gingrich’s Contract With America echoed Perot’s United We Stand. By 1996 there wasn't much left for Perot and his party to do.

Rapoport quotes historian Eric Hofstadter’s famous line on American third parties: They’re like bees. Once they’ve stung, they die.

The problem with a third-party president

America's two-party duopoly has been going on a long time. What's new is the world where even the duopoly's favored ideas are stymied. Today, the chances of infrastructure investment or immigration reform aren't much better than the chances of single-payer health care. What's changed isn't that Washington is closed to new ideas. It's that it's closed to any ideas.

Could a third party break that deadlock? Probably not. In fact, it might well make it worse.

Imagine a third party that actually elects a new president. Right now, the basic problem in American politics is that one of the two major political parties has an interest in destroying the president. As incoming Senate Majority Leader Mitch McConnell said in 2010, "the single most important thing we want to achieve is for President Obama to be a one-term president."

The statement is often used to paint McConnell as uniquely Machiavellian, but in truth, it's banal: the single most important thing any minority political party wants to achieve is becoming the majority party. That's not because they're evil; it's because they believe being in the majority is the best way for them to do good. But the way for them to get there is to destroy the incumbent. Hence, the gridlock we see today.

A third-party president would change this in one big way: now both major political parties would have a direct incentive to destroy the president.

"The reason congressional parties work with the president from their party is that they share policy goals and because they share electoral goals," says Sarah Binder, a congressional scholar at the Brookings Institution. "You put a Michael Bloomberg at the top and maybe they still share policy goals but they don’t share electoral goals. So you sever that electoral incentive."

In fact, the perverse reality of a third party is that the major party that agrees with it the most is also the most threatened by its existence. Think of Ralph Nader acting as a spoiler for Al Gore. So the fact of sharing policy goals often means they're directly opposed on electoral goals. No one in Congress is going to want to help an executive whose success is a threat to their chance of ever being in the majority again.

The problem with a third party in Congress

Arguably, a third party could attack congressional gridlock at its source: by winning seats in Congress and then doing … something … to fix the chamber. But that something is hard to imagine.

I asked Binder for the rosiest possible scenario for a congressional third party. But she couldn't come up with much. "I can’t even quite wrap my head around the politics, the electoral politics, the institutional politics, that would ever lead a third party to be in a position to make a difference in Congress," she said. "Everything in Congress is structured by the parties. If you want committee assignments, it’s the parties that control committee assignments. Unless you can displace a major party I don’t see how you get the toehold that gives you institutional power."

Rapoport didn't have much more of an answer. Third parties, he said, "are bad at process." They tend to be structured around a charismatic founder or a particular issue but, if they get far enough to actually wield power, they're ground to death by the byzantine institutions of American politics.

You can see that in Congress now, in fact. There are a number of third-party candidates serving in the Senate. Maine's Angus King, Vermont's Bernie Sanders, and Alaska's Lisa Murkowski were all elected as third-party candidates (Murkowski ran as a Republican write-in after losing the Republican primary). But in order to wield any power they've allied themselves with one of the two major parties. Sanders and King caucus with the Democratic Party and vote like typical Democrats — indeed, Sanders is thinking about running for president as a Democrat. Murkowski caucuses with the Republican Party and votes like a Republican. Even when Congress has three parties, it really only has two.

If a third party did win seats in Congress and accepted less institutional power for more party coherence, it's hard to say what problems it would solve. Congress is riven by disagreement and an inability to compromise. A third party would simply add another set of disagreements and another group who could potentially block action to the mix.

Which is all to say that the perverse incentives and byzantine structures that are causing so many problems for our two-party system would end up causing just as many problems, if not more, for a multi-party system. A third party might change the ideas Washington takes seriously. But it's hard to see it fixing the fact that Washington can't do much with the ideas it already does take seriously.

Correction: This post initially stated that Lisa Murkowski ran for Senate as an independent after losing Alaska's Republican primary. In fact, she ran as a Republican write-in candidate.