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.”