Marco Rubio would be a terrible president. His tax proposals make George W. Bush look fiscally prudent. He acts as if America can use sanctions, war, or the threat of war to bludgeon its adversaries into submission despite the devastating failure of that approach since 9/11. He has been dishonest and gutless on immigration. He has flirted with climate-change denial even though his hometown now regularly floods.

Still, if I lived in any of the nine Super Tuesday states that allow non-Republicans to vote in their GOP presidential primary, I would cross over—forfeiting my chance to cast a ballot for Hillary Clinton or Bernie Sanders—and vote for Rubio. Other liberals should do the same. Those who can should write him checks. Whatever it takes to stop the nomination of Donald Trump.

There are three arguments against what I’m proposing.

1. Argument number one is that, from a liberal perspective, Rubio is not the best candidate in the GOP field. John Kasich is. That’s true, but as a vehicle for stopping Trump, Kasich is useless. He has barely any money and barely any campaign infrastructure. He’s only seriously competing in his home state. At this point in the race, only one Republican has the combination of elite and mass support necessary to have even a sliver of a chance of defeating Trump. And it’s Rubio.

2. Argument number two is that Rubio’s not much better than Trump. On some issues, in fact, he’s worse. Unlike Trump, Rubio does not challenge the logic underlying the Iraq War. Nor will he call America’s campaign-finance system corrupt.

But as important as these issues are, there’s a more fundamental difference between the two candidates. Rubio respects the Constitution, and in particular, the Bill of Rights. Trump does not. Trump does not threaten American democracy. He is not trying to deny Americans the right to choose their leaders. But he does threaten American liberal democracy, the idea that there are certain rights so fundamental that even democratic majorities cannot undo them. Trump is not subverting public opinion. He’s exploiting and fomenting its basest aspects. In deciding how much to appeal to the public’s most hateful and bloodthirsty desires, most politicians exercise a measure of restraint born in part from a respect for legal norms and individual rights. Trump calls this restraint “political correctness” and flaunts his disdain.

I doubt Trump is the first post-9/11 presidential candidate to realize he could rouse a majority of Americans—or at least a majority of Republicans—by calling for the murder of terrorists’ family members and by giving a full-throated endorsement of torture. But the others stopped short. Other Republican presidential candidates have demonized Muslims, but none ever called for banning every single Muslim from entering the country. Other prominent Republicans subtly questioned President Obama’s Americanism. Trump denied he was born in the United States.

The difference is shame. It’s vaguely possible to imagine another Republican candidate launching the canard about Muslims celebrating 9/11. But only Trump—despite a thousand fact checks proving him wrong—would double down on the claim. Other Republicans have played on the right’s hatred of the mainstream press. But only Trump calls for changing libel laws to make it harder for journalists to write critical stories about him. And only Trump openly threatens the donors who fund efforts to defeat his campaign.

It’s no coincidence that Trump has praised Vladimir Putin. Although Trump probably couldn’t get away with everything the Russian leader has done, a Trump presidency could move the United States in the direction of what Fareed Zakaria calls “illiberal democracy.” Americans would still elect their presidents, but those presidents, once chosen, would face fewer restraints on their power and be free to more severely curtail the rights of targeted groups. Many of those restraints, after all, are a matter of convention. We can’t know how robust they are until someone challenges them. Trump will challenge them in ways Rubio will not.

3. The third argument against liberals supporting Rubio is that America will benefit if Trump destroys the Republican Party. If the GOP splits, or loses massively this fall, then perhaps moderates will regain influence and the Republican Party—or whatever supplants it—will stop denying climate change, stop refusing to vote on judges, and stop pushing the United States to the brink of financial default. Maybe as a result of Trump, the party’s “fever may break.”

I understand the argument’s allure, but it’s reckless. Although it’s highly likely Trump would lose a general election, there are no guarantees. Hillary Clinton could be indicted. Terrorists could strike two days before Americans head out to vote. If nothing else, the course of the presidential race so far should instill a healthy modesty in anyone inclined to make blanket assertions about what will happen in six months.

But we do know this: Once Trump is nominated, America will have crossed a line.

A man who does not respect constitutional limits and who preys upon vulnerable minorities will lead one of the two major parties. The consequences, though hard to measure, could be profound. A few days ago in Iowa, fans at a high-school basketball game chanted, “Trump,” at the opposing team, which comprised Latino, African American, and Native American players. They wielded the name of the man who could become president as a racial slur. Protesters at Trump’s rallies have been beaten. Last year, in Boston, two men beat a Hispanic man with a metal pipe while yelling, “Trump was right.” Just imagine what might happen if were Trump nominated or, God forbid, elected. In myriad ways, America would become an uglier, scarier place.

If Rubio won, by contrast, the Republican Party might be stabilized. The terms of debate between the two parties would remain roughly the same. Given how radical even “establishment” Republicans like Rubio actually are, that’s a depressing prospect.

Nominating Trump would upend all that. American politics would fundamentally change. But as conservatives have often reminded liberals, change does not equal progress. Sometimes even a morally corrupt status quo is better than what follows. A Trump nomination would represent a leap into a terrifying political unknown.

Liberals should try to forestall it by backing Marco Rubio. And if we fail, we should implore conservatives to help stop Trump by backing Hillary Clinton. Although the polarization in American politics today is vast, there are still norms that both decent liberals and decent conservatives cherish, and do not wish to see smashed. Across the ideological divide, it’s time to close ranks.