At the CNN debate in Miami, GOP candidates sparred over immigration, social security, how to talk about Muslims and more. Here are the key moments. (Sarah Parnass/The Washington Post)

THE MORNING PLUM:

At last night’s debate, Donald Trump lorded it over his rivals with supreme confidence. Gone was narcissistic, rambling, insult-spraying Trump. In his place stood calm, unifying, presidential Trump. The Donald noted with satisfaction that his foes were mostly laying off of him. “I can’t believe how civil it’s been up here,” he said, by which he really meant, “all you losers have surrendered to me, and I’m loving every minute of it.” And Trump may be right: it’s possible that by next week, he will be on a path to winning the nomination outright.

But if there is anything last night’s debate really revealed, it’s that Trump may not have any idea what is about to hit him soon enough. If Trump does become the nominee, he will run into a buzz saw of reality otherwise known as the general election, and he may not know how badly mangled he’ll get.

Last night’s debate is being widely described as a shift in tone: rather than lob schoolyard insults at each other, the GOP candidates had a real policy debate. And that’s true. But in the process, the debate really revealed the limitations to the scrutiny Trump has faced on policy in the context of the GOP primaries — and that foreshadows, by contrast, just how brutal the scrutiny of Trump on policy will be in the general election, once those limitations are removed.

Consider a few of the main attacks that Trump had to endure last night. When Trump vaguely promised to keep entitlements solvent and to cut “waste, fraud and abuse,” Marco Rubio made a spirited case against Trump’s budgetary hocus pocus, repeatedly saying the numbers “don’t add up.” But Rubio was constrained from pointing out a key reason Trump’s numbers don’t add up — Trump’s tax plan would deliver a huge, deficit-busting tax cut for the rich — because Rubio’s plan does the same thing. Democrats speaking to a general election audience will be freer to attack Trump on this front.

Rubio also had a good moment when he offered a serious critique of Trump’s anti-Muslim demagoguery. But if anything, this was a reminder that throughout these primaries, Trump’s rivals have mostly refrained from such critiques. On immigration, the GOP candidates also had an intense debate over its impact on American workers, but Republicans have laid off offering a sustained moral and substantive case against Trump’s absurd plan for stepped-up mass deportations. After all, Trump’s demagoguing on Muslims and immigrants resonates among a lot of GOP primary voters. But Democrats will be freer to attack Trump’s xenophobia frontally, and very, very hard, before a general election audience.

Or consider how climate change played. Trump has regularly offered up laughable buffoonery on the issue, claiming that the Chinese invented the concept of global warming to rip off U.S. manufacturing or insisting that U.S. action is futile because “it’s a big planet.” Last night, Rubio sounded somewhat more sensible, acknowledging the realty of climate change, but he, too, was constrained into saying that America shouldn’t act, because other countries won’t (never mind the global climate deal), since apparently GOP primary voters don’t want to hear anything else. Before a general election audience, Dems will be freer to slice Trump’s climate absurdity to ribbons.

The point is that the GOP primaries are offering Trump a kind of cosseted environment, in which the parameters of acceptable policy debate before the GOP primary audience ensure that Trump is not being subjected to anything close to the scrutiny he’ll endure in a general election, on either his specific proposals or his lack of policy depth in general.

Last night, looking towards the general election, Trump predicted that he’ll “win it easily.” He can be forgiven for believing this, given how soft the policy scrutiny has been on him thus far. But should he win the nomination, he’s in for a pretty nasty awakening.

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* GOP CANDIDATES TURN DOWN VOLUME OF CIRCUS: The New York Times notes that the Republican candidates, particularly Marco Rubio, were largely subdued in their attacks against Trump, even though the hugely important March 15th contests loom:

It was as if Mr. Trump’s rivals had decided, after so many months, that there was no upside anymore in trying to beat him at his own game. Gone were the heated interruptions, the name-calling, and…the wave of attacks about his checkered business history…Mr. Rubio only occasionally disagreed with him in the debate. It was a signal that if his campaign is nearing the end, he intends to exit the race on a higher plane.

One wonders whether Rubio, anticipating the very real possibility that his candidacy could be over after losing Florida next week, is laying the groundwork to exit on a more dignified note.

* A GOOD PERFORMANCE BY RUBIO, BUT IS IT TOO LATE? That’s the consensus of Republican strategists and activists in key primary states who spoke to Politico:

A majority of Republican insiders said Rubio won the debate. But another quarter gave the victory to a subdued Trump, who wasn’t under siege from his opponents as in the past two debates….even some insiders who said Rubio performed well also indicated that he did little to damage Trump, who is leading both public and private polling ahead of next Tuesday’s winner-take-all primary.

If Rubio does lose Florida, the second-guessing of Rubio’s decision to dive into the mud will be deafening. But it’s not clear that anything else would have worked, either.

* TRUMP’S LACK OF SERIOUSNESS IS UNMASKED, SORT OF: Dan Balz points out that the relatively subdued, more-policy-focused debate ended up revealing just how laughably shallow Trump’s grasp of policy really is:

That Trump has certain skills as a candidate is without question. He can dominate a debate or a news cycle with relative ease. His ability to keep opponents at bay and off balance has been stellar. But there is much more to being president than that, which is why there are so many doubts about him among the electorate at large. What the debates have shown is that Trump’s lack of depth on issues continues to be a key part of the story of his quest for the presidency.

Republicans may not have been able to unmask Trump’s lack of seriousness convincingly enough to matter to GOP voters.  But Democrats will be able to do it in the eyes of the general election audience.

* WHY TRUMP’S RIVALS MAY NOT CATCH HIM: Scott Bland and Steven Shepard take a hard look at the delegate math, and conclude that it’s highly unlikely anyone but Trump can win a majority and the nomination:

Barring major upsets in Florida and Ohio next week, Ted Cruz will need to win approximately 70 percent of the remaining delegates to clinch the nomination outright. That essentially makes Cruz’s path to 1,237 as unlikely as Marco Rubio’s route.

Rubio may need at least two thirds of the remaining delegates to win a majority. So the only two likely outcomes are: Trump wins the nomination outright; or there’s a contested convention.

* DELEGATE MATH COULD LOOK MORE DAUNTING NEXT WEEK: David Wasserman runs the numbers:

Heading into pivotal March 15 contests in Florida, Illinois, Missouri, North Carolina, and Ohio, Trump’s opponents’ focus has shifted to preventing him from winning a delegate majority. But to do so, they will actually need to beat him somewhere on Tuesday. Right now, Trump would need to win 53 percent of remaining delegates to reach a majority. But if he were to sweep all five states, he would only need about 43 percent of delegates thereafter to clinch the nomination.

A sweep seems somewhat unlikely; Trump may lose Ohio to John Kasich. Rubio could pull off a surprise in Florida. But it’s possible that by next week, Trump’s lead could look insurmountable

* KEEPING THE CANDIDATES HONEST ON TRADE: Paul Krugman asks whether the success of Sanders and Trump show that we are in a “protectionist moment,” and argues for a serious, non-demagogic, reality-based assessment of the impact of trade deals:

There’s a lot of speculation that Mr. Sanders may have gained traction by hammering on the evils of trade agreements. Meanwhile, Donald Trump, while directing most of his fire against immigrants, has also been bashing the supposedly unfair trading practices of China and other nations….Mr. Sanders is demagoguing the issue, for example with a Twitter post linking the decline of Detroit, which began in the 1960s and has had very little to do with trade liberalization, to “Hillary Clinton’s free-trade policies.”

It seems feasible, though, that Sanders’s attacks on Clinton over trade could continue resonated, particularly in states like Ohio, which votes next Tuesday.

* AND BEN CARSON IS BACK, THIS TIME TO ENDORSE TRUMP: Later this morning, Ben Carson will endorse Trump at a press conference:

The support of Carson, a famed retired neurosurgeon and author, will likely give Trump a boost with GOP base voters and evangelicals, who embraced Carson’s campaign in its early days and fueled his brief rise to the top of Republican primary polls….according to people close to him, Carson has gradually come to see Trump as the GOP’s best chance of winning a general election and turning out droves of disengaged voters.

Carson seems reality-challenged on a whole lot of fronts, so perhaps it’s not surprising that Trump’s general election viability is one of them.