In the five weeks since the last Republican presidential debate, terrorists have struck in Paris and San Bernardino, California, Donald J. Trump has proposed barring Muslims from coming to the United States, Ben Carson's support has evaporated and Sen. Ted Cruz of Texas has emerged as Trump's stiffest competition. Sen. Marco Rubio of Florida has gone on the attack against Cruz, and Gov. Chris Christie of New Jersey, who has less money than many senators up for re-election, has shown signs of life, at least in New Hampshire.
All of which is to say that Tuesday night's debate at the Venetian Resort Hotel Casino in Las Vegas will showcase a markedly changed race.
Cruz, who is showing signs of consolidating support among hard-line conservatives, is likely to come under sharp attack. Issues like national security and immigration will take on greater urgency. And candidates who are struggling to leapfrog into contention are sure to try to seize on one of the last ripe opportunities to do so before voters in the early nominating states begin making up their minds.
But while much has changed, one figure remains the same: Trump, the leader in national polls and the field's chief provocateur and pacesetter, whose outrageous and sometimes demagogic comments have not only roiled the race but also amplified divisions between party leaders and the conservative grass roots.
Nowhere has that been more evident than with Trump's call last week to bar Muslims from entering the country, an inflammatory idea that his rivals will almost certainly have to respond to at the debate.
None of them will second Trump's proposal. But the degree to which they repudiate him, or else delicately try to tap into the same fears in a more considered way, will illustrate how they are positioning themselves with Trump's supporters.
With nearly 6 in 10 Republicans backing the proposal, but about the same number of Americans overall opposing it, according to a new Washington Post-ABC News poll, the rest of the field must weigh how to avoid appearing out of step with Republican primary voters without appearing to support discrimination in a way that could do grave damage to their prospects in a general election.
"The problem for the other candidates is that Trump is initiating and shaping the conversation on national security at a time of great anxiety, and they're forced to react," said Kevin Madden, a Republican strategist not affiliated with any candidate, who called Trump's proposal "offensive, unconstitutional and entirely unworkable."
"The key is for the other candidates to say, 'We need to do a better job making our immigration and visa system safer, and here's an alternative that isn't rooted in something that would be viable,'" Madden said.
The sharpened focus on national security and immigration will also illuminate tensions among Christie, Cruz and Rubio, while putting immense pressure on Carson.
Rubio and Cruz have been locking horns for weeks on immigration and national security, and they are likely to clash Tuesday. With Cruz surging into first place in two recent Iowa polls and terrorism emerging as the top issue, Rubio is likely to step up his assault on Cruz for his support of guest-worker visas and of restrictions on some domestic surveillance programs.
Cruz has responded by highlighting Rubio's work on a 2013 effort to offer immigrants in the country illegally a pathway to citizenship — a major vulnerability for Rubio among conservatives.
In Tuesday's debate, Cruz will almost certainly invoke "amnesty" to bludgeon Rubio, if given the chance. Remarkably, Rubio's role in the bipartisan effort to overhaul immigration laws has not been brought up once in the previous four debates.
Rubio, though, would most likely counter — or try to pre-empt such an attack — by questioning Cruz's somewhat muddled views about what to do with unauthorized immigrants already in the country. And Trump, who advocates mass deportation, could join in the pile-on, now that Cruz has emerged as his biggest threat.
Rubio and Cruz can also expect to come under attack from Christie, who has staked his fortunes on doing well in New Hampshire. There, Rubio, who is averaging second place in the polls, poses an obstacle to be overcome.
"He's never here," Christie said of Rubio while campaigning in that state over the weekend. "Come on, Marco."
Rubio's supporters include some of the center-right voters to whom Christie is making a case. And both are focused on a muscular response to the rising threat of Islamic State terrorism.
Cruz, meanwhile, has proposed "carpet-bombing ISIS," using an acronym for the Islamic State, but his support for stricter surveillance laws and his limited experience as a first-term senator will probably invite criticism from Christie, who has begun more sharply contrasting his experience as a federal prosecutor with the senators in the race.
Then there is Carson, the retired neurosurgeon, who took the lead in some national polls this fall but has since fallen sharply amid questions about his grasp of national security. He could fade from contention for good if he is not able to speak authoritatively about how he would confront the threat of terrorism.
Nick Ryan, a Republican strategist in Iowa who is advising the super PAC supporting Mike Huckabee, said Cruz could face the most difficult outing.
"I would expect that he and Rubio will tussle a little bit," Ryan said. But the big question, he said, is Trump, who until now has appeared to pull his punches against Cruz.
"Sometimes he appears willing to attack Cruz, other times he wants to pretend that they're best buddies," Ryan said of Trump. "And I think if Trump wants to dispatch him, then he better engage with him."
That would present Cruz with a possibly pivotal choice: whether to keep turning the other cheek to a front-runner he has sought to embrace, or to strike back against the candidate who could be standing in the way of his nomination.