Jeffrey Phelps / AP

Nine Republican candidates will squeeze onto a Las Vegas debate stage on Tuesday, but the lights seem to be shining brightest on two of them at the moment: Donald Trump and Ted Cruz.

While Trump has expanded his lead in national polling to new heights over the last two weeks, Cruz has surged in Iowa and edged ahead of the billionaire boaster in the crucial first-in-the-nation voting state. Vying for the same set of conservative voters, the two for months have had something of an informal nonaggression pact. But that has frayed in the last few days, and it may be shredded in Las Vegas.

First, Trump seized on reports that Cruz criticized him in private and referred to the uncompromising Texan as “a little bit of a maniac.” Cruz initially responded in jest to Trump’s taunts, tweeting clips from Flashdance and Tommy Boy. But as a former college debating star occupying center stage for the first time on Tuesday night, he might have something bigger planned for Trump.

While the last GOP debate took place just over a month ago, the contest in Las Vegas—airing on CNN at 8:30 p.m.—will be the first since the terror attacks in San Bernardino. And it will occur on the same day that a bomb threat caused officials to shut down the entire Los Angeles public school system. Moderator Wolf Blitzer will undoubtedly press both Trump and his rivals on Trump’s widely-condemned proposal to bar Muslims from entering the United States. And they will—hopefully—be forced to speak with more clarity and specificity on what other measures they would take to protect the homeland and how, or whether, they would expand the war President Obama launched against the Islamic State in Iraq and Syria.

What about the seven other candidates on the main stage? Marco Rubio and Chris Christie have also climbed in the polls over the last several weeks, apparently benefitting both from solid previous debate performances and from the heavy recent focus on foreign policy and national security. Yet while both are contending for a strong finish in New Hampshire (and Iowa for Rubio), they are nowhere close to Trump’s level of support. Neither anymore is Ben Carson, the neurosurgeon who briefly topped Trump in the polls earlier in the fall but who, by his own admission, has struggled to demonstrate that he has the foreign-policy chops to win the voters’ trust as commander-in-chief. Cruz has supplanted Carson as the main conservative threat to Trump both nationally and in Iowa.

Tuesday night will be the fifth Republican primary debate, and the challenge for any of the second- and third-tier candidates may be one of familiarity: Now that voters have seen them repeatedly and gotten used to their debating styles, it may be harder for any one performance to move the needle. That is likely to be especially true for John Kasich, Carly Fiorina, and Jeb Bush, who have seen little upward movement in the standings for months. (Bush has already acknowledged that debates are not his strongest format.) Rand Paul barely made it into the main debate this time at all, and he is now at the unenviable stage where his campaign was forced to deny reports that he would drop out of the race. Expect some taunting from Trump on that front, although Paul’s differing views on foreign policy could liven up the debate over the right balance between national security and privacy right.

It may have long since lost its novelty, but there will once again be an undercard debate starting at 6 p.m. featuring Mike Huckabee, Rick Santorum, Lindsey Graham, and George Pataki. Fairly or not, these long shots will be looking for a viral moment or two just to capture enough attention to climb back into the campaign.

During the prime-time contest, one of the more interesting dynamics will be whether the trailing candidates go after Trump or Cruz. Attempts to take down Trump have generally backfired, and rivals like Rubio, Christie, and Bush might try instead to rattle Cruz and leapfrog him as the chief alternative. Rubio and Cruz have already been jousting over their votes on legislation to limit the NSA, while a play by Christie against Cruz’s hard-right positions could appeal to independents who can vote in the New Hampshire primary.

Six weeks remain until the Iowa caucuses, and the seemingly invincible—and extraordinarily healthy—Trump has held the top perch for months. The lectern beside him, however, has been a revolving door of challengers. Cruz will have his turn on Tuesday night, but at least for contenders like Rubio and Christie, there’s plenty of time for another shakeup.

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