Tuesday's CNN Republican debate was a last chance to shine for candidates in 2015. It was a crucial clash before the race goes into the deep freeze over the Christmas and New Year holidays, with just seven weeks to go before the Iowa caucuses.
And as the first GOP showdown since the terror attacks in Paris and San Bernardino, California, revealed the alarming, expanded reach of ISIS, the showdown in Las Vegas offered a preview of how a Republican president would reshape U.S. national security policy.
Here are five takeaways from a fiery night in Sin City.
It was the showdown everyone wanted to see: Ted Cruz and Marco Rubio, the two young princes of right-of-center politics, feuding over the conservative crown.
Cruz has positioned himself as the tea party outsider and evangelical favorite, while Rubio is now tops in the polls among more establishment favorites. The confrontation could not be put off any longer.
The two clashed on immigration, data sweeps by intelligence agencies and regime change in the Middle East, and both drew enough blood to allow their camps to claim victory -- and to set up tantalizing future duels.
Cruz, basking in his ride to the top of the Des Moines Register/Bloomberg Iowa poll over the weekend, came prepared to deflect Rubio's attacks. "Marco knows what he is saying is not true," Cruz said, shrugging off an assault by his rival from Florida over HIS congressional vote to limit mass data surveillance programs used by U.S. intelligence agencies to track terror suspects.
In an intervention aimed at the party's grass-roots, Cruz cited an article by radio talk show host Mark Levin in his defense -- implicitly making the point that he, and not Rubio, is the most authentic conservative.
And for good measure, when Cruz accused Rubio of backing an amnesty bill, he again reached for a conservative icon, quoting President Ronald Reagan saying, "there was a time for choosing."
But Rubio stood his ground, and appeared to score points when he skewered Cruz on his exact position on offering a path to legal status for illegal immigrants during a congressional battle on the issue in 2013.
Rubio has consistently argued that Cruz did support such a move -- a charge that could weaken the Texas senator among core conservatives who see such support as amnesty.
"I have never supported legalization, and I do not intend to support legalization," Cruz replied, taking a more concrete position on the issue than ever before.
Trump: I'm a Republican
Six months into his campaign, Donald Trump broke some news: he's definitely running to be the Republican candidate for President.
The GOP front-runner has frequently teased the GOP's top brass with the possibility that he could mount an independent presidential run if he is not treated fairly. Such a maneuver would likely ensure Democrats keep the White House, which Salem Radio's Hugh Hewitt noted when he asked if Trump would stay in the party.
Trump said he's happy in the GOP.
"I really am, I'll be honest. I really am," Trump said. "I am totally committed to the Republican Party. I feel very honored to be the front-runner."
That should at least reassure GOP leaders who fear that Trump could walk with his fired up constituency of angry blue-collar voters and fatally harm the party's chance of capturing the White House. But it won't ease the fears of some party members who worry that Trump, with his explosive rhetoric and calls for foreign Muslims to be banned from U.S. soil, is causing serious damage to the party's image.
As far as the debate, Trump probably didn't win -- but he didn't have to. The reality show star has based his campaign on his mastery of the airwaves and social media, so is in less need than other candidates of splashy highlight reel moments in a debate. He can simply call up a television anchor and hijack the news agenda.
He did appear to come off second best, for once, in a clash with Bush -- though nothing so far in the campaign has loosened Trump's bond with his faithful coalition.
"I'm at 42, and you're at 3," Trump snapped at the former Florida governor, comparing poll numbers as Bush started to annoy him.
CNN political commentator S.E. Cupp said that Trump had turned in a "poor performance" but with the caveat that his supporters probably would not care.
But Trump's struggle with details was on show as he tried to explain his comments that America should "shut down the Internet" to fight terrorists and came under fire from his rivals for his call to target the families of terrorists in the Middle East, in apparent contravention of the laws of war.
Jeb Bush has a pulse
"I had fun," Bush, the one-time establishment favorite and long-ago GOP front-runner, told CNN's Jake Tapper after the debate -- and it actually seemed like he was telling the truth.
After a series of tepid debate performances, which coincided with his campaign's tumble from the top tier and saw him bullied by Trump and mercilessly pummeled by his one-time protégé Rubio, the former Florida governor finally put up a fight.
Bush even managed to get under Trump's skin for the first time and has a case that he emerged from a bout with the bombastic billionaire with a win -- or at least a draw -- on points.
"Donald, you are not going to be able to insult your way to the presidency," Bush warned in a testy exchange, which prompted Trump to mock his rival's single-digit poll numbers and efforts to raise the energy levels that Trump has blasted as lacking throughout the race.
"A little of your own medicine there, Donald," said Bush clearly enjoying the chance to rattle his tormentor as he got on Trump's nerves by trying to interrupt him.
If Bush has any chance of a getting back in the mix, he needs to prove not just that he is the most qualified potential commander-in-chief, but also that he is tough enough to take down Trump — a feat no other Republican has yet achieved.
Despite coming away from the exchanges with Trump on top, and putting in the strongest debate of his campaign, the question remains: Is anyone still listening? Despite a massive war chest, Bush is languishing at 3% in national polls.
His attacks on Trump seemed to encapsulate the frustration he has endured in the campaign.
"This is not a serious kind of candidate," Bush said after lambasting Trump's call for foreign Muslims to be temporarily banned from entering the United State. As he spoke, the bafflement that Republican voters aren't getting his argument was clearly written on the face of the son and brother of presidents.
"Donald, you know, is great at the one-liners, but he's a chaos candidate. And he'd be a chaos president. He would not be the commander in chief we need to keep our country safe," Bush said, trying again.
GOP prime-time debate verbal attacks:
It's scary out there
If viewers buy into the apocalyptic Republican worldview on display in the pair of GOP debates, they might be frightened to venture out of their front doors.
"We have people across the country who are scared to death," New Jersey Gov. Chris Christie warned, habitually staring straight into the camera directly into America's spooked living rooms.
Tuesday's clash was the first Republican debate since terrorist attacks orchestrated or inspired by ISIS in Paris and California shocked the world and transformed the tone of the 2016 campaign, so it was not surprising the showdown bristled with hawkish rhetoric.
But even so, it was a dark, frightening world that the candidates portrayed.
"America has been betrayed," Christie warned, blaming President Barack Obama and likely Democratic nominee Hillary Clinton, as he seized on a bomb threat to Los Angeles schools earlier Tuesday that appeared to be a hoax. "Think about the mothers who will take those children tomorrow morning to the bus stop wondering whether their children will arrive back on that bus safe and sound."
Christie clearly sees the ominous resurgence of jihadist terrorism represented by ISIS as a political opening for him to highlight his credentials as a terrorist-hunting former prosecutor — a possible lifeline as he grasps for a lifeline for a campaign stuck in single digits in the polls.
All of the candidates were under pressure to prove to conservatives that they were tough enough to be commander in chief.
The spine-chilling rhetoric was not just for effect -- it offered a preview of what is certain to be an important theme in November's general election.
"This President hasn't kept us safe," said Rubio, encapsulating that theme, in a debate constantly punctuated by candidates vowing to keep Americans and their children safe.
Bush warned that America must "destroy ISIS before it destroys us," as he joined the chorus that ISIS represents an existential threat to America's very survival.
"The war that we are fighting now against radical Islamist jihadists is one that we must win. Our very existence is dependent upon that," said retired neurosurgeon Ben Carson.
If anything the picture was even darker in the undercard debate as former Pennsylvania Sen. Rick Santorum warned: "We have entered World War III. World War III has begun."
Sen. Lindsey Graham
didn't mince words either. "We're at war, folks," he said. "They're not trying to steal your car. They're trying to kill us all."
Referring to ISIS martyrs, Graham said: "They're ready to die. Bring on the virgins."
Was there a commander-in-chief on stage?
Each candidate made a case that they were uniquely qualified to lead the nation in a perilous era of foreign wars, resurgent terror threats and rising powers like China that are determined to challenge U.S. primacy.
Rubio, who bills himself as a foreign policy expert, delivered a comprehensive tutorial on nuclear policy. Cruz vowed to "utterly destroy ISIS" and stop lone wolf, home-grown terror attacks before they happen. Ohio Gov. John Kasich complained that world leaders just wrapped up a summit on climate change in Paris and not one on terrorism.
Christie blasted Obama as a "feckless weakling" and said the last thing America needed was to pick a president from the squabbling senators on stage. But he may have undermined his case with a slip of the tongue, saying he was ready to stand with "King Hussein of Jordan." The current monarch is the deceased Hussein's son, Abdullah II.
While candidates strove to outdo one another in hawkish rhetoric, a serious, substantial discussion did break out over foreign policy.
As candidates like Trump and Cruz weighed in against regime change in places like Libya and Syria, others like Rubio and Kasich took a more interventionist approach, laying bare an ideological fault line dividing the Republican Party on national security.
The clashing viewpoints did not just shape the last GOP presidential debate of 2015. Which side wins the argument could dictate whether the party rallies around a nominee in the style of George W. Bush or nestles closer to the more limited view of America's footprint abroad preferred by Obama.