Will Donald Trump force Republicans to admit we can do worse than Hillary Clinton?

Here's a truth partisans are not supposed to admit: Whether the president is a Democrat or Republican, America usually remains more or less the same place.

Policy changes matter, and therefore it matters who wins elections.

But the institutions that underlie our government and our society matter more than which party steers those institutions at a given time.

Because of this truth, there are more important things than which party wins a presidential election. Leaders who threaten to undermine those fundamental institutions - even if they are within your own party - are worse than normal politicians from the other party. But that's a damaging admission that party-loyal politicians really, really prefer not to have to make.

The Donald Trump campaign is like an experiment - seeing whether a GOP candidate can be so manifestly threatening to the institutions that prevent American society from falling apart that he will get Republican officials to admit there are worse outcomes than electing Hillary Clinton.

That breaking point hasn't arrived yet for most Republicans.

Marco Rubio, for example, has called Trump an "erratic individual" and a "con artist" who "is wholly unprepared to be president of the United States" and "has no ideas of any substance on the important issues." But he says he's standing by a pledge to support Trump if he's nominated, despite all that.

So this week, Trump has ratcheted up the test: Will normal Republican officials continue to insist that a candidate whose supporters have committed repeated acts of violence, and who defends by his staff when they allegedly do so, would make for a less horrifying president than Hillary Clinton?

On Tuesday, Trump's campaign manager allegedly roughly grabbed the arm of Breitbart News reporter Michelle Fields and yanked her away from Trump as she asked a question, leaving her with bruises, according to Fields and Washington Post reporter Ben Terris. The Trump campaign responded with a denial and attacks against Fields' trustworthiness, even though Politico obtained an apparently corroborating audiotape.

On Wednesday, an attendee apparently sucker punched a young black man, a protester who was being escorted out of a Trump rally. Local police proceeded to detain the man who had been punched, only arresting the alleged perpetrator after the assault got news coverage. The alleged attacker, 78-year-old John McGraw, told Inside Edition he enjoyed "socking the hell out of that big mouth" and "next time we see him, we might have to kill him."

Obviously, Trump did not instruct McGraw to commit assault. But at a rally last month, he did say he'd like to punch a protester in the face, like you could in the "good old days" when such a protester would "be carried out on a stretcher." And at another rally last month, he encouraged attendees to "knock the crap out of" any protesters who brought throwing tomatoes, saying he'd pay the legal fees of anyone who did so.

Trump spokeswoman Hope Hicks released a statement saying only "we are not involved." Trump surrogate Andy Dean (a former contestant on "The Apprentice," of course) told CNN on Thursday the punch was "impressive" for a 78-year-old and looked like "good exercise."

Trump has created a campaign culture that joyfully celebrates violence. From the attacks against protesters, to threatening to kill the families of terrorists, to insisting the military will commit war crimes if he instructs them to do so, he is explicitly rejecting the institution of rule of law that underlies American happiness and prosperity.

This should be scarier than a president who won't repeal Obamacare.

It is not literally impossible to conceive a Republican candidate too horrible for Republicans to endorse. In 1991, when white supremacist leader David Duke was the Republican nominee for governor of Louisiana, Republicans all the way up to President George H.W. Bush endorsed his opponent, who was not only a Democrat but widely considered to be corrupt.

"Vote for the crook, it's important" went the slogan for supporting Edwin Edwards over Duke, and it worked - Duke still won a solid majority of white voters, despite the Republican establishment falling away from him, but he lost the election.

So, this is a question that needs to be put repeatedly to non-Trump Republicans: Has Trump reached David Duke levels of unacceptability? How about now? What about now?

It was the last question at last week's debate, and it deserves to be asked again tonight of Rubio, Ted Cruz and John Kasich: After what we've seen this week, do you really still stand by your pledge to support this man for president?