There will be seven men and one woman on stage in Milwaukee on Tuesday night for the fourth Republican presidential debate. But the story of this debate — and every debate — is of head-to-head matchups, those moments when two candidates square off and one wins. Like "Thunderdome," but for politics.

Below are the three most important face-offs I expect to see and why they matter. Don't forget it all gets underway at 9 p.m. Eastern time!

The entire narrative of the last GOP presidential debate was written within the first 20 minutes when Jeb Bush tried to take on his onetime protégé Marco Rubio for missing lots and lots of votes in the Senate. Rubio was waiting for the attack and pounced — destroying Bush with a rapid-fire set of facts about past nominees and their missed votes and painting the former governor as a puppet of political consultants urging him to go on the attack.

The leak to the New York Times on Monday of a plan from Bush's super PAC, Right to Rise, to spend upward of $20 million bashing Rubio as dangerously inexperienced and a major risk for the party in a general election seems to be aimed at making clear to the former governor the path he needs to take if he wants to win. And, by and large, the super PAC is right: There is not a path to the nomination for Bush that doesn't go through — and, really, over — Rubio.

Here are the problems for Bush: 1) He's not super comfortable going on the attack (and by "not super comfortable," I mean incredibly uncomfortable) and 2) He has said many nice things about Rubio in the not-so-distant past.

Bush simply can't lose this matchup as badly as he did in the last debate. If he does, talk that he just can't do what he needs to in order to win will grow. A lot.

Donald Trump doesn't understand Ben Carson's appeal. Like, at all.

"This is a strange election, isn't it?" Trump said Monday night in Springfield, Ill. "If you try and hit your mother over the head with a hammer, your poll numbers go up." His amazement at Carson's rise doesn't change the fact that the retired pediatric neurosurgeon is now at the top of the GOP field.

Trump has, until the past week or so, played nice with Carson. It's hard to see that continuing during Tuesday's debate unless Trump totally disavows all of the bad things he has said about the doctor on the campaign trail over the past few days. Less interesting to me than the Trump attack – you sort of know what you're going to get there — is how Carson responds. In the first three debates, Carson has avoided attacking any of his rivals and also has faced almost no attacks from them. But, in this debate, Carson will be one of the prime targets — if not the prime target. Can he effectively counter-punch?

Ask a smart D.C. Republican whom she thinks the last two Republicans standing will be and the answer you will get most often is Rubio and Ted Cruz.

The thinking goes like this: Rubio is already starting to consolidate the establishment behind him, and Cruz will eventually emerge as the outsider choice once Carson and Trump fade. What's interesting is that Rubio and Cruz have barely mentioned each other in the three debates leading up to Tuesday night. But Cruz's decision to cancel campaign events last month to return to fight a quixotic battle against the debt and budget deal seem to be the senator from Texas laying the groundwork for a full-frontal assault against the absentee Rubio.

That fight may not happen on Tuesday night. Regardless, watch how the two men act toward each other given their status as the likeliest survivors of the GOP contest.