5 things to watch in tonight’s debate Marco Rubio, Donald Trump and Ben Carson at the start of the third Republican Presidential Debate. | Getty MILWAUKEE – Nearly two weeks after a climactic battle in Boulder, Republican candidates take the stage tonight for a high-stakes clash that will shape the contours of the winter primary season. Jeb Bush, who limped out of the last debate, is under perhaps the greatest pressure of all. With his poll numbers plummeting to the mid-single digits, the former Florida governor must prove he can handle his rival and one-time friend, Marco Rubio. Ben Carson, who has been under intense scrutiny in recent weeks over his personal narrative, will need to fend off an almost-certain offensive from Donald Trump. Chris Christie, a once-towering figure who rose to prominence through his larger-than-life persona, aims to demonstrate he can make a splash in the less-watched and lower-profile undercard debate. And host Fox Business wants to prove it can put on a better show than competitor CNBC, which fumbled badly with its Republican debate last month. Here is what POLITICO is watching in Tuesday’s Republican debate. A Bush-Cruz tag-team? Jeb Bush and Ted Cruz are starkly different candidates – one an establishment favorite with a patrician pedigree, the other a tea party bomb-thrower who’s made a career out of antagonizing leaders of his own party. Yet, on Tuesday night, they may end up being debate partners. Campaign insiders are abuzz about the possibility that the two candidates, who are running in opposite lanes of the primary, could team up to take down Marco Rubio. The thinking? Rubio is a competitor to both, making a play for the establishment vote Bush is seeking and the conservative support that Cruz is trying to lock up. Both have gone after Rubio in recent days. Bush has hammered the Florida lawmaker for missing Senate votes, while Cruz, during an interview on CNN last week, chose the ever-loaded term “moderate” to label him. Aides to Bush and Cruz declined to discuss the possibility of a debate alliance – and in the end, it may not pan out. Cruz, after all, has so far been reluctant to go after his rivals during debates. Bush, though, has given no indication he plans to let up. Even after his boss’s botched attack on the senator during the last debate, Bush campaign manager Danny Diaz, an operative steeped in the art of oppo research, has privately expressed a desire to continue jabbing. Rubio’s team says he’ll be ready: The senator has been holding debate practice sessions led by longtime adviser Todd Harris. The former Florida governor is under pressure to perform, no matter how he decides to approach Rubio - something his advisers are well aware of. On Wednesday, Diaz, along with Bush fundraiser Heather Larrison, held a conference call with New York donors to discuss the path forward. At one point, according to one person on the call, Diaz brought up Bush’s less-than-impressive performance in the last debate. Bush, Diaz said, was preparing for this round differently: He said the former governor was now getting help from veteran Republican image guru and Roger Ailes adviser Jon Kraushar - and said supporters should expect a better performance. Trump vs. Carson Those who’ve followed Donald Trump’s debate style closely have noticed a distinct pattern. Near the start of each debate, the real estate developer and television celebrity will open with a “big blast” against one of his rivals. In the first showdown, it was directed at Rand Paul. In the second, at Jeb Bush. And in the third go-round, at John Kasich. This time, his target seems certain: Ben Carson. In recent days, Trump has been torching Carson – who has passed the billionaire in some early state polls – over his personal narrative. Appearing on CNN’s “State of the Union,” on Sunday, Trump raised questions about details of a childhood story Carson told about once stabbing a friend. He also expressed concern about the retired neurosurgeon's mentality, saying he had “pathological” anger problems in his younger years. But taking on Carson isn’t without risk. Fox Business is allowing candidates 60 seconds to rebut attacks – double the amount of time given during the last debate. That means Carson would have a substantial amount of time to push back against any criticism directed his way – and to reiterate, as he did over the weekend, his contention that questions about his past are generated by a press corps with a vendetta against him. Carson’s camp says it’s not sure what Trump will do. "You never know which Donald is going to show up,” said Carson campaign manager Barry Bennett. “The man on Twitter or the guy who tells you privately how much he likes you." If Carson does come under assault, another candidate making a play for his conservative supporters - perhaps Cruz - could come to his defense. Fox Business’s time to shine Nearly two weeks after CNBC stumbled, Fox Business, a newer and less established business network with a smaller viewership, has an opportunity to prove itself as a debate host. Those who have been briefed on the network’s plans say it wants its debate to be what CNBC’s wasn’t: Policy-focused and chaos-free. One senior Fox Business official said the network was spending the final hours in the run-up to the program carefully scrubbing questions to ensure they “squeezed out nonsense and bias.” The network’s planning team – which has been led by Fox News political director Cherie Grzech – has earned high marks from senior campaign aides. On Wednesday, Grzech helped to lead a conference call for senior campaign aides that had none of the backbiting and drama that defined the planning calls leading up to CNBC’s debate. On Monday, she led the campaigns on a walk-through of the stage – and elicited none of the complaints that arose during the tour of the CNBC site, when aides griped about uncomfortable greenroom conditions. Grzech has won plaudits for making herself accessible, handing out her email and cell phone and making it clear that she’s the point person in the event aides have concerns about how much speaking time their candidates are receiving during the program. During the CNBC debate, some aides groused they didn’t know who to turn to when problems arise – leading some to mistakenly reach out to NBC officials, who had nothing to do with the broadcast. At one point, Bush’s manager Diaz got into a mid-debate confrontation with a CNBC producer over his candidate’s allotted time. Fox Business is formatting the program to ensure that moderators have greater control. The network will be using a buzzer when the debaters reach the end of their answer time, forgoing the flashing light that CNBC utilized – which candidates blew right through. No matter what transpires, don’t expect the campaigns to target Fox Business as they did with CNBC. The parent network, after all, is a favorite of the Republican primary voters the candidates are trying to appeal to. Christie’s opportunity Chris Christie’s failure to get on the primetime debate stage is a serious blow to his candidacy, which had been picking up steam as of late. Tuesday’s undercard program, in which he will appear on the same stage as back-of-the-pack candidates Mike Huckabee, Rick Santorum, and Bobby Jindal, will draw far fewer eyeballs than the later one. Yet those close to Christie – who was described as surprised by Fox Business’ decision to keep him out of the main event - insist it could give him the chance to stand out. With fewer people on stage, they say, he’ll have a bigger platform and more time to talk. For Christie, who entered the primary as an underdog and has been trying to rehabilitate his battered political image, debates have been a strength. He’s been practicing with a tight-knit group of advisers that includes Maria Comella and Russ Schriefer. Perhaps the biggest question, though, is whether being relegated to the undercard will dry up Christie’s fundraising, which had already been lagging. The governor has maintained a small but loyal base of financial support, thanks in large part to his past tenure as Republican Governors Association chairman. He is slated to hold around a dozen fundraisers through the end of the year, including one in Northern California next week hosted by Hewlett Packard executive Meg Whitman. Bobbie Kilberg, a Christie fundraiser, insisted the demotion wouldn’t hurt the governor with the money crowd. The announcement, she contended, had sparked the interest of donors who viewed the decision as unfair – some of whom had previously said they were reluctant to support Christie. “The Fox decision on the debate is outrageous and unfair, but it is turning out to be an opportunity for Chris,” Kilberg said. “It has sparked additional focus on Chris, including by donors.” .