CNN will host the fifth round of Republican debates tonight at 6:00 and 8:30 p.m. Eastern. This could be the most contentious and lively of the main debates so far. Cruz and Rubio have been bashing one another for weeks over immigration, surveillance, and foreign policy, and Cruz is now threatening to take the lead in Iowa from Trump. Christie has recently been criticizing Rubio (mocking his lazy campaigning, questioning his “depth,” etc.), so there could be some sharp exchanges between those two as well. Rubio, Cruz, and Christie have done fairly well for themselves in previous debates, and barring some fluke they should be expected to turn in the best performances. Of those three, Rubio may have the most at stake since his campaign depends heavily at this point on getting good debate reviews. Kasich, Carson, and Fiorina will also be there, and they’ll have to turn in strong performances to halt their slide into irrelevance. Trump will presumably keep boasting about his poll numbers and belittling his rivals, since this seems to be what has worked for him in the past.

Bush and Paul have their work cut out for them to show that they are still in the mix, and Paul’s support in the national polls is low enough that he risks missing the main debate stage the next time around if things don’t change. Paul may be able to use the foreign policy fight between Cruz and Rubio to his advantage, and Rubio’s habit of dismissing any foreign policy view he doesn’t like as “isolationism” potentially sets up a chance for Cruz and Paul to team up against Rubio. On any NSA surveillance questions, Christie’s authoritarian instincts make him a natural ally for Rubio, but Christie otherwise has every incentive to beat up on Rubio, who is now his main “establishment” competition in New Hampshire.

The most significant disputes among the candidates should be over Syria policy and immigration. Rubio has managed to get through four debates without being challenged on his support for the Gang of Eight Senate bill, and I doubt he’ll be able to get through another one without coming under fire for that. Trump, Cruz, and Paul all have incentives to go after him explicitly on this point, and I can’t believe that none of them will bring it up. A debate on Syria might be instructive if it forces supporters of a “no-fly zone” to defend their reckless proposal, but it could just as easily devolve into a shouting match over whose policy is closer to Obama’s. That may be an opportunity for Cruz and Paul to use Rubio’s support for the Libyan war against him, but so far he hasn’t had to answer for his own foreign policy record at these events. It will be interesting to see how and whether Rubio can respond effectively to attacks from rivals not named Bush.

Since this will be the last main debate before the holidays, this will probably be the best chance for many of the candidates to make a favorable impression on a national audience before Republican voters stop paying attention to the race for a while.

The earlier debate will include Pataki, Santorum, Graham, and Huckabee. That’s really all that needs to be said about this one. All of these candidates should drop out and spare us any more of these “undercard” events.

I’ll be covering tonight’s debates on Twitter (@DanielLarison), and I’ll have a write-up here on the blog either later tonight or early tomorrow.