Why the Republican race is looking more and more like Donald Trump vs. Marco Rubio

Donald Trump won the Republican primary in South Carolina, which is clearly bad news for GOP elites. But zoom out on the night, and the picture looks a little bit better for them.

Specifically, Marco Rubio now seems increasingly likely to be the last candidate standing against Trump — especially now that Jeb Bush has quit the race.

We don't yet know whether Rubio or Cruz finished second in the Palmetto State (they're effectively tied). But it doesn't really matter. Cruz's strategy hinges on winning the South. So losing to Trump by 10 points in what was supposed to be Cruz country — Southern, conservative, and evangelical — is a big blow to his campaign.

And many large Southern states will start voting very soon. In just 10 days, Georgia, Tennessee, Alabama, Virginia, Oklahoma, and Arkansas will all vote on Super Tuesday, alongside Cruz's home state of Texas and a few states from other regions. If Trump can replicate his South Carolina success in many of those other Southern states, it's very hard to see a path to victory for Cruz.

Rubio's path is easier to imagine. While, unlike Cruz, he hasn't yet won a state, many of the biggest blue- or purple-state delegate prizes — like New York, Pennsylvania, and California — vote in April or later. It's hard to see Cruz being competitive in the Northeast or on the West Coast, but if Rubio consolidates the mainstream Republican vote, he could have a shot at beating Trump head to head in states like those.

Still, there are a lot of problems that could lie ahead for Rubio's campaign. So here are three big questions about this burgeoning Rubio-versus-Trump scenario — both about whether it will happen and whether Rubio can win it if it does transpire.

When, exactly, is Rubio going to win a state?

Rubio has done a good job spinning a third place in Iowa and an effective tie for second in South Carolina as wins for his campaign. But if you want to be the Republican nominee, you eventually have to win states.

And it's very unclear right now when Rubio will manage to put some real points on the board. Polls show Trump up big in Nevada (though it is a caucus state, so maybe Rubio can pull off a surprise if he has superior organization there). Most of the Super Tuesday states are Southern, so Trump or Cruz is expected to win those. And a few other Super Tuesday states are Northeastern, which Trump could well win, since he did well in New Hampshire.

Surely, one might think, Rubio would win his home state of Florida, a big winner-take-all delegate prize set to vote on March 15. Yet polls in January showed Trump up anywhere from 12 to 32(!) points there. One has to expect that Rubio will come back to win Florida, but if that's his first victory in a state, does he really have a path to the nomination?

Will the other candidates quit before Rubio falls badly behind Trump in delegates?

Jeb Bush's decision to quit the race Saturday night was a good sign that, at long last, the GOP mainstream is consolidating around Rubio.

But there's still one lingering establishment loose end, and his name is John Kasich.

Most observers think Kasich is drawing votes away from Rubio. And he could well pick up some of Jeb Bush's supporters too, now that Bush is out.

Kasich's advisers are vowing that he'll stay in the race despite his poor performance in South Carolina. He did, after all, finish second in New Hampshire. So his strategy going forward is to essentially ignore the South and instead target Super Tuesday Northeastern states, the March 8 Michigan primary, and his home state of Ohio (which holds a winner-take-all contest on March 15).

The Ohio governor's strategy is particularly problematic for Rubio because more than half of overall delegates will be awarded by the time the dust settles on March 15. So unless Kasich's support plummets before then, he could well draw enough votes to prevent Rubio from winning some states, or to push him under the threshold necessary to get delegates in other states.

Then there's Cruz. Even if Trump beats him in many of the Southern Super Tuesday contests, Cruz could perform strongly enough to win second place in many of them. And many of these states allot a large share of their delegates by congressional district — three per district, with two going to the winner, one to the second-place finisher, and zero to whoever gets third.

So if Cruz prevents Rubio from taking second in many of these states, he could make the Florida senator miss out on lots of Southern state delegates — effectively putting Rubio even further behind his true rival, Donald Trump. And with so many delegates already allotted by mid-March, Rubio would face an uphill struggle to close the gap.

If the race is Trump vs. Rubio, can Rubio actually beat him?

Republican elites are pretty confident that Rubio can beat Trump one on one, and that makes sense. Rubio's favorable ratings among Republican primary voters are far higher than Trump's. And he's led Trump in some head-to-head polls.

But Trump hasn't seriously attacked Rubio yet.

When Ben Carson looked like a threat to Trump, Trump took him down. When Jeb Bush looked like a threat to Trump, Trump took him down. And when Ted Cruz looked like a threat to Trump, Trump took him down.

Yes, it's true that Jeb Bush's Super PAC has already spent millions trying to attack Rubio, to little apparent avail. But Trump will be far more unscrupulous once he sets his sights on Rubio. He will be willing to play to GOP base voters' worst ethnic and racial fears. He will bring up Rubio's brother-in-law, who was convicted for cocaine trafficking. He will publicly spread the rumors about Rubio that Bush's team just whispered about privately. And, of course, there's that matter of immigration reform, which Bush couldn't attack Rubio on.

Point is, things will get very, very ugly. We don't yet know whether Rubio can beat Trump in a knife fight. But, to win the nomination, that's what he's going to have to pull off.

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