This post is part of Polyarchy , an independent blog produced by the political reform program at New America , a Washington think tank devoted to developing new ideas and new voices.

Last week, I wrote a piece that asked, “Will Republicans impeach Trump?” The short answer: unlikely.

In this piece, I want to explore how much this “unlikely” conclusion flows from the zero-sum logic of our two-party system. The short answer: a lot.

Because of the two-party system, Republicans are stuck with Donald Trump. If he goes down, they go down with him. There’s now no way for Republicans to advance conservative policy goals without also advancing Trump. And In this era of bipolar two-party tribal politics, no matter what Trump does, there’s always one thing worse for Republicans. Something even more unthinkable, something even more existentially frightening than Trump with his hand on the nuclear codes: Democrats having power.

In two-party politics, a “pathological liar” is always better than a Democrat

Most congressional Republicans never wanted Trump as their standard-bearer. They still don’t. But their fates are now tied to him. If Trump goes down in a dramatic impeachment (is there any other kind?), Republicans almost certainly lose their House majority in the 2018 midterms, and probably continue to suffer the repercussions in 2020. And there’s a real risk that if Trump goes down, he tries to take all the furniture with him, fracturing the Republican Party.

So Republican congressional leaders are stuck. The only thing worse than having Trump as their unpopular standard-bearer is losing power and popularity because they tried to remove him as their standard-bearer.

We saw this in the presidential election. One by one, Republican leaders who had criticized Trump decided to forget all the things they said about Trump being a con artist, utterly amoral, a pathological liar. The most important thing was preventing Hillary Clinton from becoming president. Donald Trump was a vote against Hillary Clinton. There was no alternative, only a binary choice.

Many voters felt the same way. They had reservations aplenty about Trump. But right-wing propaganda told them to remember “all the children Hillary Clinton has personally murdered and chopped up and raped.” Some were convinced that “Clinton will demolish America as we know it, and every human life will be under government control.” As talk show host Dennis Prager put it recently in a National Review article (italics mine): “Many conservatives, including me, believe that it would have been close to over for America as America if the Republican candidate, who happened to be a flawed man named Donald Trump, had not won.”

Whether motivated by power or by the existential righteousness of their moral cause (or more likely, by a complicated, self-justifying mix of both), Republican leaders continue to make the same craven calculus: It’s always better to dissemble than let Democrats have an edge. No matter what Trump does, no matter what line he crosses, they’ve made the same choice again and again. In two-party politics, a “pathological liar” is always better than a Democrat.

But what if some congressional Republicans pushed to impeach Trump?

Let’s posit that a sizable portion of congressional Republicans prefer to have Donald Trump out of office. What would happen if they joined with Democrats to call for Trump’s impeachment?

Many of these Republicans would likely find themselves threatened in primaries, treated as traitors to the Republican Party. And some of them would probably lose.

Let’s say that these Republicans were even willing to go a step further. Let’s say they believed Trump was destroying the Republican Party as they knew it, and it was time to let the party die and start a new party in its place. Call it the “Conservative Party.” Though they would disagree with Democrats on most issues, the danger of Trump’s reckless presidency would be one point of agreement.

The Republican Party would then split. How would this play out?

For sake of simplicity, let’s say half of today’s Republican voters would stick with the Trump Republican Party, while the other half move over to the new Conservative Party (which would stand for more traditional conservative values of restraint and limited government).

So assuming a 50-50 electorate right now, we’d instead now have a 50-25-25 electorate (Democrat-Republican-Conservative).

Now, assuming Democrats don’t also split 50-50, districts would now be 50-25-25 districts (50 percent Democrats, 25 percent Republicans, and 25 percent Conservatives), and 60-40 districts would become 40-30-30 districts. Democrats could even win 66-34 Republican districts (34-33-33).

The result? Democrats would win a landslide in the House, easily taking 300 seats. They’d also probably win a bunch of Senate seats too.

This would be obviously unfair, since half the country wanted a left-leaning party and half the country wanted a right-leaning party. But because the right couldn’t agree on the party, the left won a landslide.

That’s how our single-member, plurality electoral system works. And it’s why Republicans would never do this — because they could anticipate exactly what would happen. If the Republican Party splits over Trump, Democrats win.

You can quibble with my guesstimates of percentages and breakdowns here. Maybe “Republicans” and “Conservatives” wouldn’t compete against each other in every close district. Maybe there’d be three different right parties. But even if it only happens in a handful of districts, even if only a limited number of Republicans break off to stand against Trump, they could tip control to Democrats.

The point is this: In our two-party system, even Republicans who oppose Trump face intense pressure to stay on the team. If they defect, or harm the Republican brand, they could tip a few elections to the other side. And in our knife’s-edge, winner-take-all system, this would make all the difference.

What if we weren’t stuck in a two-party system?

Let’s imagine that the US somehow moved to a proportional voting system, in which the parties would get the seat shares proportional to their votes. As a result, some Republicans would now be free to form a new anti-Trump conservative party, giving voters the option of supporting a conservative party without necessarily helping Democrats.

Or, even if we kept the single-winner district system and just added ranked-choice voting, right-of-center voters could still pick the between two types of Republicans They could rank these two Republicans first and second, knowing that even if their preferred right-of-center standard-bearer lost, that wouldn’t necessarily mean the Democrat won.

This would have two obvious benefits:

1) Right-of-center politicians could hold Trump accountable without tipping power Democrats.

2) Conservative voters aghast at Trump’s activities could also hold Trump accountable without helping Democrats.

In short, the election could then be two separate referenda — one on Trump, and one on conservative policies. Voters would not have to accept Trump in order to get conservative policies, if that’s what they wanted.

Deeper into the widening gyre of two-party politics

Unfortunately, that’s not how our electoral system works. Because we have single-member plurality-winner districts, Republican leaders who formed a third party to support conservative policies but oppose Trump would be handing congressional elections to Democrats.

In the two-party system, Republicans in Congress need Trump as much as he needs them. As a result, Republicans, whether or not they think Trump is a narcissistic, amoral pathological liar, are stuck with him. So are voters who want conservatism without Trump.

Going forward, this trap will only make politics more ugly. Think about how this will influence the tenor of the 2018 midterms, especially since midterm elections are even more about base mobilization than presidential-year elections.

Democrats will almost certainly run it as a referendum on Donald Trump, doubling down on all the scariest apocalyptic narratives they can find to boost turnout. Donald Trump as evil villain is the great unifying force of the Democratic Party now. In many ways, this is a tremendous gift to Democrats, not only because Trump is unpopular but also because it spares Democrats having to figure out a real economic message that would cause some heartburn among competing factions within the party.

For Republicans, the challenge will be to keep their troops feeling certain that however imperfect Trump might be, Democrats would by definition be worse — that it really might be the end of the republic if Republicans lose the house. This likely means doubling down on all the aggressive us-against-them white Christian identity politics and apocalyptic narratives they can find to make sure their base shows up.

And so deeper into the widening gyre we go. This is the logic of our two-party system right now. And Donald Trump is still our president, leading us into deeper tribalism as he takes advantage of our two-party system’s fatal flaw.