BY BENJY SARLIN, NBC News
LAS VEGAS (NBC News) — The last Republican presidential debate was barely a month ago, but it feels like a lifetime.
Since the field last gathered in Milwaukee in November, the world has endured terror attacks in Paris and San Bernardino, California, and secured a sweeping climate agreement. In the GOP primary, Dr. Ben Carson has all but collapsed in Iowa, and that has helped push Texas Sen. Ted Cruz into the spotlight. Donald Trump's escalating rhetoric against Muslim-Americans, meanwhile, has roiled the race while bolstering his own standing with GOP voters nationally.
It's a lot to digest, and CNN's debate Tuesday in Las Vegas — the last of the year — looks to be a memorable one. Here are a few of the angles we'll be watching.
What to do about Trump?
Since the last debate, Trump's campaign has erupted into open bigotry, including phony tales of American Muslims celebrating 9/11, fake statistics slandering black Americans as violent and finally a call for a total ban on Muslim travel into the United States.
With emotions high after the deadly attacks in Paris and San Bernardino, Republican candidates and conservative commentators are concerned that Trump's rabble rousing could drag the GOP into a dangerous place. And, as Trump learned when he planned — then abruptly cancelled — a trip to Israel after criticism from Prime Minister Benjamin Netanyahu, his Muslim-bashing could have real consequences in the international arena.
Will candidates confront him if he offers more of the same on Tuesday? Will anyone be able to get a word in edgewise over Trump's outrageous claims? How will Republican voters, who polls show are closely divided on Trump's proposed Muslim ban, respond to high-profile debate over it on Tuesday?
The national security debate
Setting aside Trump, the recent terrorist attacks have refocused the Republican primary conversation around national security and it's an area where the candidates have significant differences.
Florida Sen. Marco Rubio, former Florida Gov. Jeb Bush and New Jersey Gov. Chris Christie have tried to stake out ground as the field's hawks and have each attacked Cruz and Kentucky Sen. Rand Paul as weak on national security. Rubio, in particular, has targeted Cruz's vote to reform the NSA surveillance program revealed by Edward Snowden, which collected data about phone calls (but not the content of the calls themselves). Cruz and other supporters of the legislation argue that it put the program on sounder legal footing without harming intelligence gathering.
The bigger argument, though, concerns the candidates' overall policy vision. Borrowing from Paul's libertarian circles, Cruz has accused Rubio and Bush of sowing conflict in the Middle East by supporting interventions in Libya and Syria to overthrow military strongmen rather than restricting their focus to radical terrorist groups like the Islamic State, which Cruz has pledged to "carpet bomb."
Rubio, however, has accused Cruz of being an "isolationist" and he and Bush have argued that eliminating the Islamic State requires removing Syrian dictator Bashar al-Assad from power in order to wind down that country's civil war. Bush, who has called for greater use of ground forces in Iraq and Syria, has said Cruz's emphasis on air power is simplistic and "not a strategy." Expect these divides to come up in a major way on Tuesday.
Trump vs. Cruz
Cruz has been moving into a decisive lead in Iowa polls. Trump really, really, really does not like it when other candidates do that. Not surprisingly, there's been some tension between the two lately.
The twist here is that Cruz, unlike the rest of the field, has spent the last six months showering Trump with compliments to avoid exactly this confrontation. That strategy held up until this week, when The New York Times reported Cruz had questioned Trump's "judgment" as commander-in-chief at a private fundraiser.
That was enough of a green light for Trump, who seemed relieved to finally have an excuse to go after his new top rival. Trump said on "Fox News Sunday that Cruz was a "a bit of a maniac" in the Senate and tweeted he was disappointed Cruz would "speak behind my back."
Cruz has tried to keep the truce up anyway and his pandering is growing more cringe-worthy by the day. The debate is a whole other story, though. Will Cruz stand up for himself if Trump targets him onstage? Will Trump, the most famous birther in America, renew his old doubts about the Canadian-born Cruz's eligibility for the presidency?
The not-so-green party
The world is celebrating a landmark climate change deal this week that represents years of work by America, European allies, rising powers like China and India, rivals like Russia and dozens of smaller nations to prevent environmental ruin.
No one in the Republican field seems to care much, though, and a number of candidates — including Cruz and Trump — are openly hostile to the idea man-made climate change even exists. To the extent candidates have discussed the issue, it's mostly been to jeer at regulations on carbon emissions, downplay hopes of international cooperation and mock the issue as a distraction from terrorism.
Even putting aside the overwhelming scientific consensus on climate change and the policy debates about how to stop it, the Paris agreement is going to be a critical component of American foreign policy for the next president. Will candidates offer any positive ideas as to how they'll meets the deal's ambitious goals - or how they'll manage the diplomatic fallout if they abandon it?