Aside from Trump, one of the men standing directly next to him on stage — Bush — will be the candidate to watch Thursday. Even though Trump has shot to the top of the polls, Bush is still generally seen as the front-runner for the GOP nomination, meaning Trump and many of the other candidates on stage will place a giant bull's-eye on the former Florida governor's back. Bush's goal will be not to let Trump (or anyone else, for that matter) drag him into the mud, and to use the real-estate mogul's presence as an opportunity to look like "the adult in the room" with a serious vision. Bush needs to defend against attacks on his record, particularly on immigration and Common Core, while maintaining the high ground.
Walker, who will be standing on the other side of Trump on stage, will have a similar mission to Bush: staying above the fray. Trump has recently taken aim at Walker's economic record in Wisconsin, but the governor has said that he doesn't plan to take on anyone, including Trump, and instead intends to focus on his accomplishments in office. As a top-tier candidate nationally and the current front-runner in Iowa, though, Walker will likely face criticism from his other rivals as well. Some conservatives have accused him of flip-flopping on hot-button issues like immigration and abortion, and Walker has been preparing his response should that come up during the debate. If he can convincingly deflect those criticisms, he will emerge from the debate in good shape.
Despite the recent firestorm over Huckabee's comments likening the Iran nuclear deal to the Holocaust, Huckabee placed fourth in Fox's average of recent national polls. The 2008 Iowa caucus winner remains in good standing with social conservatives, but the competition for that slice of the electorate is much fiercer now than it was eight years ago. Huckabee doesn't necessarily need to go on the attack Thursday night, but he needs to remind those evangelical voters why they supported him back then and make the case that he better represents their values than some of the new faces that have broken on to the scene since.
Carson will be one of the biggest question marks on the debate stage. As a retired neurosurgeon who has never held elected office, he's the only candidate, aside from Trump, who enters Thursday with no previous campaign debate experience. Carson's stump speeches are often long and meandering, so his challenge will now be condensing those stem-winders into short sound bites that will have some staying power after the debate ends. If Carson can deliver even just one memorable line, while avoiding any Nazi or slavery comparisons, the debate will be a success.
Cruz may be one of the few candidates on stage Thursday who doesn't need to worry about Trump attacking him. Both candidates are vying for the same group of tea party-aligned, antiestablishment voters, but Cruz has continued to defend Trump even as he's made controversial comment after controversial comment. Cruz won't be able to play nice forever, and if Trump's candidacy lasts into the fall, Cruz will need to make an assertive effort to win over some of the billionaire's supporters at some point in order to survive. Cruz doesn't need to worry about doing that just yet, but he should at least start laying the groundwork Thursday for that eventuality.