At the Republican debate Thursday night in Miami, Donald Trump somehow managed to sound like the most presidential candidate on the stage. He was clearly aiming for that, toning down his usual manic and rambling affect. He sounded calm and measured, sort of, and even like he had practiced his opening and closing statements.

Still, Trump coming across like the most presidential guy on stage was very much in spite of his performance rather than because of it. He repeatedly blundered and had to talk around his obvious policy ignorance, and at one point was badly caught out by Marco Rubio. But on the whole, none of the anti-Trump faction managed to sustain a serious critique of the frontrunner, because they were too undercut by their own party's dogma (or their own oleaginous personality, in the case of Ted Cruz). And on the most serious issue of the night — the escalating violence at Trump's rallies — they completely failed.

Rubio was the only one to land a clean hit on Trump, with an assist from moderator Dana Bash, who kept pressing Trump on whether he would reverse President Obama's re-opening of diplomatic relations with Cuba and close the U.S. embassy there. Trump obviously had no idea what to say, and Rubio's follow-up taunts gave him the best applause line of the night.

Something similar happened on the issue of anti-Muslim bigotry, where Rubio tried briefly to stand up for basic democratic values of equal rights and equal treatment. It was a nice moment, but badly undercut by him (plus Cruz and John Kasich) then going on at length about how they were going to bomb half the Middle East, tear up the Iran nuclear deal, and continue helping Israel bludgeon the Palestinians.

On multiple occasions, Cruz made it reasonably clear that Trump is simply blowing smoke. But he couldn't drive the point home properly. Partly it's that own arguments were sprinkled with non sequiturs and utter nonsense (like the Smoot-Hawley tariff causing the Great Depression), but more importantly, his odious weasel personality undermines the substance. Before criticizing Trump, he would sigh theatrically, criticize the position, and then obliquely undermine Trump by suggesting that we need a president who "understands the issues," or a similar formulation — the implication being that he, Cruz, is the smartest guy in the room and that everyone already knows it. For those of us who aren't already Cruz admirers, it makes you want to drop a caterpillar down the back of his suit. (However, it's important to note that there are obviously quite a lot of Cruz supporters, and he is increasingly obviously the only plausible not-Trump.)

Kasich was more upbeat and reasonable-sounding, providing a contrast to Trump even though he did not challenge him outright. But his foreign policy ideas are so bananas — he seemed eager to provoke a shooting war with both China and Russia — that it undermined his credibility.

But it was the violence question on which the three Trump challengers showed their true colors: abject cowardice. The violence at Trump rallies, a longstanding feature of the campaign, has escalated of late. A black protester was allegedly sucker-punched by an elderly Trumpist on Wednesday. Inside Edition found the assailant, who said he liked "knocking hell out of that big mouth...next time we see him, we might have to kill him." (He was later charged with assault.)

It's not just random Trumpists either. Trump's own campaign manager Cory Lewandowski allegedly attacked a reporter from Breitbart, according to the Washington Post's Ben Terris, who was present at the time. He allegedly seized her arm hard enough to leave marks and knocked her down after she asked a question. Fields tweeted a picture of her bruised arm, and Politico later obtained audio backing up Terris' version of events, which the Trump campaign denies.

CNN's Jake Tapper presented a crushing indictment of Trump by simply quoting his own words back at him encouraging his crowds to attack protesters. Trump basically shrugged, saying he did not condone violence but that some protesters were "bad dudes" who were "big, strong, powerful guys doing serious damage."

The reaction from the challengers was bupkis. None of the other three candidates tried to nail Trump on his creeping fascism, or even to firmly establish that beating people up is beyond the pale of democratic values. Instead they pivoted to their own shtick, even implying that the violence was in some ways justified.

This was the last debate before a series of enormous primaries on Tuesday, including in Kasich's home state of Ohio and Rubio's home state of Florida, when Trump may well lock up the nomination. If he does, it will be because the Republican Party resolutely failed to confront Trump and everything he represents. Instead, they allowed him to plausibly present himself as presidential.