Fox Business will host the sixth main Republican debate tonight at 9:00 p.m. Eastern. The pointless “undercard” debate will air at 6:00. The main debate will have the fewest candidates it has had yet. The field has been reduced to Trump, Cruz, Bush, Rubio, Kasich, Christie, and Carson. Tonight is likely to be the most contentious debate thus far, as all of the candidates have ongoing feuds with two or more of their rivals.

The main clash tonight will likely be between Cruz and Trump following their recent quarrels about Cruz’s place of birth, Trump’s “New York values,” and Cruz’s criticism of Trump’s qualifications on foreign policy. Rubio will be caught up in disputes with Christie, whom his campaign has been attacking in New Hampshire, and with Bush, whose allies have been savaging Rubio for his opportunism on immigration. Rubio and Cruz will probably also get into more fights over immigration and foreign policy, but Rubio will have his hands full with the many “establishment” foes that he has to fend off now. Kasich has an incentive to take Rubio down a peg or two if he can, since he is his closest “establishment” rival in New Hampshire. Carson will be present, but that’s about all there is to expect from him at this point.

Rand Paul and Carly Fiorina have been excluded from tonight’s event. The debate will be less interesting because of Paul’s absence. Paul’s campaign has struggled over the past year, and his polling early states hasn’t been that great in recent months, but his presence in the debates has been a useful and much-needed corrective to the party’s prevailing hawkish views that virtually all of the other candidates accept in its entirety. He has also done good work in pushing back on the bad arguments put forward by Rubio and Christie on surveillance. Paul’s arguments with other candidates haven’t done him much good in terms of winning support, but he has done the party and the country a service by challenging some of the most reckless and irresponsible policies favored by his rivals. He has made it harder for the most aggressive candidates to hide their policies behind boilerplate and euphemisms, and that helps voters have a better understanding of what these candidates want to do. Paul won’t be participating in the “undercard” debate. That’s just as well, since I have no desire to watch another one of those anyway.

As always, Rubio needs a strong debate performance, since his campaign has been relying heavily on the debates to boost him in the early states. The first voting is just a little over two weeks away, and Rubio hasn’t been able to break out above 12-15% anywhere, so he needs to distinguish himself tonight more than usual. Trump and Cruz have the most to lose by quarreling with each other, but it seems inevitable that each has to go after the other in the final weeks before Iowa. Until now, they have avoided serious clashes with each other during the debates, and that has allowed both to benefit the most from the debates. Rubio’s “establishment” rivals have their last chance to make an impression, but because they will all be attacking Rubio at least some of the time that last impression may be very negative. Previous debates have not done much to shake up the overall order of the field, and tonight is unlikely to be any different. Even so, most of the candidates need to have their best debate tonight if they want to have an impact on the race beyond the end of February.

P.S. As usual, I’ll be covering the main debate on Twitter (@DanielLarison) tonight. I will not be covering the “undercard” debate.